Cathode Materials for Future Electric Vehicles and Energy Storage

Advanced and safer lithium-ion battery based on sustainable electrodes. Xiang Ding , Xiaobing Huang , Junling Jin , Hai Ming , Limin Wang , Jun Ming. ...
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Cathode Materials for Future Electric Vehicles and Energy Storage Systems

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cathode materials because it ensures high energy density, good capacity retention and rate capability, and facile synthesis and electrode fabrication. Its high electric conductivity is another advantage for use under low-temperature operation. Excellent cycling performance is usually seen for LixCoO2 (x = 0.5−0.6) up to 4.3 V; above this voltage, Co dissolution and a large anisotropic structural change cause a phase transition from hexagonal to monoclinic.6 This mechanical failure results in rapid capacity fading. Even more critically, an exothermic decomposition reaction occurs at the delithiated state (LixCoO2, x < 0.5) above 200 °C, which releases oxygen from the crystal structure;7 this can compromise the cell safety. Partial doping of Co sites by heteroelements8,9 and nanolayer surface modifications with electroinactive moieties10 can be applied to mitigate these concerns. However, it is still challenging to use LiCoO2-based cathode materials for EVs and energy storage applications due to their insufficient thermal properties. Padhi and Goodenough5 also suggested a new type of threedimensional olivine (i.e., LiFePO4) in 1997. This material employs the low-cost and earth-abundant element iron. Due to the presence of P−O covalent bonds in the crystal structure, this material intrinsically suffers from low electric conductivity. However, the application of carbon coatings on LiFePO4 enabled it to overcome the low electric conductivity and allowed for operation at higher rates.11 A flat discharge plateau is observed at approximately 3.5 V. Due to the contribution of the carbon coating, the carbon-coated LiFePO4 could deliver a discharge capacity close to its theoretical capacity (approximately 170 mAh g−1), which is higher than that of LiCoO2 in terms of the gravimetric capacity. In consideration of the volumetric capacity in terms of the tap densities of active materials, which are about 1.5 g cm−3 for LiFePO4 and 2.9 g cm−3 for LiCoO2, the resulting capacity is still lower than that of LiCoO2. Additionally, the presence of P−O bonds in the crystal structure guarantees thermal stability at a highly delithiated state, which negates the intrinsic low conductivity concerns. However, a serious issue was raised by Koltypin et al.,12 namely, 40% of the atomic iron was dissolved after storage for 20 days at 60 °C. Amine et al.13 also demonstrated the dissolution of ∼600 ppm Fe2+ from noncoated or carboncoated LiFePO4 after aging for a week. Note that the dissolution of Fe2+ is far greater than the dissolution of Mn2+ (∼60 ppm) from LiMn2O4 under the same testing conditions. More seriously, LiFePO4 shows very low capacity at low temperatures below 0 °C,14 which is a very important concern for large-format applications. Mn doping or Mn-based LiFePO4 is interesting because of the high redox potential of Mn2+/3+

he continued release of more and more greenhouse gases, which have led to global warming, is a serious issue that must be resolved on the global scale. One of the reasons behind this increase in pollution is the indiscrete use of fossil fuels, which are mainly used for industrial and transportation applications. Renewable energy sources represent alternatives to fossil fuels, although they are intermittent. This emphasizes that selecting an appropriate energy storage system is very important in order to successfully utilize renewable energy. In consideration of the weight, storage capability at high rates, space/size limits, and durability, rechargeable lithium-ion batteries (LIBs) are the best candidate for storing energy that is produced from renewable energy sources. Since the commercialization of LIBs in 1991 by Sony, rechargeable LIBs have become ubiquitous and are used in many applications ranging from portable electronics to electric vehicles (EVs). At the cell level, lithium-ion cells are mainly composed of a cathode, anode, electrolyte, separator, and case. Among these major components, the cathode is particularly important because the operation voltage and capacity directly affect the energy density of cells and batteries. The energy density at various temperatures, particularly at low temperatures, is also an important issue because LIBs are sometimes used when the temperature falls below 0 °C. Therefore, selection of the cathode material is a key parameter when building reliable batteries for large-format applications such as EVs and energy storage (Figure 1). Let us briefly take a look at some representative cathode materials: LiCoO2,1 LiNiO2,2,3 LiMn2O4,4 and LiFePO4.5 Since being introduced by Mizushima and Goodenough et al.1 in 1980, LiCoO2, which possesses a two-dimensional O3-type layer structure, is still one of the most popular and suitable

Received: February 17, 2017 Accepted: February 20, 2017

Figure 1. Schematic illustration of the future EV. © XXXX American Chemical Society

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DOI: 10.1021/acsenergylett.7b00130 ACS Energy Lett. 2017, 2, 703−708

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ACS Energy Letters (around 4 V).15 However, Mn2+ dissolution and the far lower electric conductivity of these materials diminish the merits of the LiFePO4 backbone. However, good thermal stability and moderate cycling rate performances have enabled the adoption of carbon-coated LiFePO4 as a cathode material for LIBs to power EVs in China. Because of the superior high rate performances (even at low temperatures)16 and excellent thermal stability,17 spinel LiMn2O4 cathode materials have attracted interest for use in large-format applications. Thackeray et al.18 discovered that lithium ions can be inserted and extracted into/out of the spinel framework. One of the unique features of these materials is that they have two voltage plateaus at around 4 V. However, the main problem is capacity fading at elevated temperatures due to disproportionate Mn dissolution, as proposed by Hunt et al.:19 2Mn3+ → Mn2+ and Mn4+. Many works have been done in attempts to improve the cycling stability of LiMn2O4 by substitution20,21 and surface modification.22 It is clear that partial doping of Mn3+ sites in LiMn2O4 is very effective in suppressing the cooperative Jahn−Teller distortion in the octahedron Mn3+O6; thus, the resulting capacity retention can be significantly improved. Also, surface modification with functional nanolayers is effective in minimizing side reactions with the electrolyte, especially for scavenging HF that is generated as a byproduct of electrolytic salt decomposition.23 Although the aforementioned structural and electrochemical properties were substantially improved, the main drawback of the spinel LiMn2O4 is its low capacity (148 mAh g−1 in theory), compared with those of LiCoO2 and LiFePO4. Again, the voltage plateau was observed at a high voltage (around 4 V), which is an obvious advantage of the spinel LiMn2O4. Provided that Mn3+ can be fully replaced by an electroactive species (e.g., Ni2+) that allows the total average oxidation state of Mn to be 3+, the crystal structure and two-electron reaction by Ni2+ and Ni4+ can be maintained, enabling a theoretical capacity of 148 mAh g−1 at 4.7 V. Spinel Li[Ni0.5Mn1.5]O4 was discovered by Amine et al.24 in 1996, where the oxidation state of Ni and Mn are 2+ and 4+, respectively. However, they focused on Li+ insertion into Li1+δ[Ni0.5Mn1.5]O4, which offers a voltage plateau near 3 V. A year later, Zhong and Dahn et al.25 retested the spinel Li[Ni0.5Mn1.5]O4 in the voltage range of 3− 4.9 V. They found that the voltage plateau appeared at 4.7 V, which is ascribed to the two-electron reaction driven by Ni2+/4+ redox, leading to their discovery of a new layer compound of Li[Ni1/3Co1/3Mn1/3]O2.26 Ideally, the absence of Mn3+ in the Li[Ni2+0.5Mn4+1.5]O4 compound relieves the disproportionate Mn3+ dissolution. Notwithstanding, the long-term cycling performance is still questionable due to oxidative decomposition of the electrolyte during operation at high voltages. Further elaboration is required to determine suitable electrolytes for high-voltage applications in order to utilize these highvoltage cathode materials. Because of the high power at various temperatures (even below 0 °C) and extraordinary thermal stability of LiMn2O4, this material is usually blended into the cathodes of LIBs, especially those used in EVs and energy storage devices. LiNiO2 does not exist in a stoichiometric form, and its derivatives are isostructural with LiCoO2; these are known to deliver higher capacities above 150 mAh g−1 in the voltage range of 2−4.3 V.27 Most cases show Li1−xNi1+xO2 forms, which result from cation mixing at Li sites upon the formation of Ni2+.28 Ohzuku et al.29 emphasized how important it is for the synthetic method to first have electrochemical activity. They

also suggested that both cationic displacement and drastic variation in the c-axis, resulting from consecutive phase transitions from H1 to H3 phases at the highly delithiated state, must be overcome in order to obtain stable cycling performance. Arai et al.30 successfully overcame the abovementioned difficulties by partial doping of Ni3+ with Co3+ and Mn3+. They also mentioned the importance of excess lithium for producing phase-pure Co- and Mn-doped LiNiO2, of which the obtained discharge capacities were approximated to be 200 mAh g−1. A wide range of solid solutions were introduced in both Co- and Mn-doped Li[MxNi1−x]O2 (with values of x as large as 50%). Note that Co doping in LiNiO2 improved the electrochemical performances due to the smaller degree of cation mixing,31 while Mn doping deteriorated the electrochemical performances but greatly suppressed the exothermic reaction at the highly delithiated state.32 Formation of the solid solution LiAlO2−LiNiO2 is possible due to the structural similarity of both materials. Furthermore, due to the strong Al− O bond in Li[AlxNi1−x]O2, the anisotropic c-axis variation can be dramatically suppressed by sacrificing the capacity, which results from the electrochemical inactivity of Al3+.33 Other dopants in transition metal layers are also available to stabilize the crystal structure of LiNiO2, including Ti,29,34 Mg,35 Fe,36 and Zn,37 which aim to reduce cation mixing in Li layers. Summarizing the above-mentioned information, Li[CoxNi1−x]O2, Li[MnxNi1−x]O2, and Li[AlxNi1−x]O2 compounds can be hybridized as two types: Li[Ni1−x−yCoxAly]O2 (NCA) and Li[Ni1−x−yCoxMny]O2. A new approach has resulted in the formation of a solid solution between Li[CoxNi1−x]O2 and LiAlO2 end members, bringing about NCA. As mentioned above, Al3+ is electrochemically inactive, while the added Co3+ contributes to the capacity (instead of Al3+); this leads to a capacity of approximately 200 mAh g−1 at both room temperature and 60 °C for Li[Ni0.8Co0.15Al0.05]O2. A large amount of dopant leads to better thermal stability but also promotes a capacity decrease and poor electrochemical performance due to the electroinactive species. Structural evolution from the hexagonal 1 phase to the hexagonal 2 phase is dominant upon charging and discharging. This reversibility is responsible for the good capacity retention of Li[Ni0.8Co0.15Al0.05]O2 in coin cells. Recently, Watanabe et al.38 pointed out the importance of DOD (depth of discharge) control for long-term cycling for 2500 cycles when set to 60% DOD. A morphological change is a possible cause of this capacity fade. In particular, particle cracking has been associated with anisotropic changes in the lattice, which results from repetitive extraction and insertion of Li+ ions, as suggested by Miller et al.39 Namely, each primary particle is always exposed to the electrolyte. Indeed, LiPF6based electrolytes always contain a small amount of water as an impurity. Water is mostly propagated via the decomposition of LiPF6 salt, especially at elevated temperatures. Consequently, the existence of a small amount of water causes breakdown of the electrolyte and accelerates HF generation. The produced HF continuously attacks the active materials, and the transition metal ingredients are gradually dissolved into the electrolyte; these tend to adhere to the surfaces of the cathode and anode. As a result, NiO and Li2CO3 are formed on the surface of the active materials and along the grain boundaries; this raises the cell impedance due to the lower electrical conductivity between active materials, resulting in capacity fading. However, the release of oxygen from the crystal structure at the highly delithiated state (i.e., Li1−δ[Ni0.8Co0.15Al0.05]O2) is inevitable at 704

DOI: 10.1021/acsenergylett.7b00130 ACS Energy Lett. 2017, 2, 703−708

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ACS Energy Letters temperatures above 180 °C, which is the onset temperature of the exothermic decomposition reaction. More efforts should be made to minimize oxygen evolution from the crystal structure by increasing the Al content, although the capacity will be sacrificed as a result. Dahn’s group reported the solid solution Li[MnxNi1−x]O2 (x < 0.5), and Spahr et al.40 found the compound Li[Ni0.5Mn0.5]O2, although they believed that the average oxidation states of Ni and Mn were both 3+. Additional work done by Ohzuku et al.41 demonstrated that the oxidation states of Ni and Mn are 2+ and 4+, respectively, and the compound is activated by the Ni2+/4+ redox couple while Mn4+ remains inactive but plays a role in retaining the crystal structure to minimize the c-axis variation (Figure 2). Again, due to the presence of tetravalent

of Ni-rich materials. These are spontaneously exposed to the moisture in air, causing the formation of LiOH and Li2CO3 on the surface of the particles. This phenomenon is observed to be more dominant as the Ni content increases. The residual lithium compounds are concentrated in the range of 6000− 25000 ppm on the surface of particles. These lead to negative effects on the slurry preparation and casting process on a larger scale, causing gelation of the N-methyl-2-pyrrolidone (NMP) solvents. However, the most serious problem encountered with residual lithium compounds is that they decompose at high voltages to produce dangerous gases. The effect of CO2 adsorption on Ni-rich Li1+zNi1−x−yCoxMyO2 (M = Al, Mn) was studied by Takeda et al.45 Their results suggest that the CO2 absorptivity increased as the ratio of Ni3+/Ni2+ increased. In the case of the Li1+xNi0.80Co0.15Al0.05O2 composite, CO2 was adsorbed very readily because the Ni3+ content is almost 100%; this led to the poor electrochemical performance of the composite. Xiong et al.46 attempted to decrease the amount of residual lithium compounds by simply washing the Ni-rich Li[Ni0.8Co0.1Mn0.1]O2 composite with water. Their results showed that washing had a positive effect on the cycling performance and structural stability of the composite. Another attempt to reduce the number of residual lithium compounds involved reheating the active materials at moderate temperatures, as suggested by Jo et al.47 After reheating Ni-rich Li[Ni0.7Mn0.3]O2 powders at 200 °C, the material showed a higher capacity with better cycling performance. Reheating caused the amount of residual lithium compounds on the surface to be significantly lowered due to the evaporation of water molecules. However, although the reheating process is effective, it should be done very carefully because it may damage the crystal structure if the heating temperature is too high; elevated temperatures can increase cation mixing in the Li layers. To improve the capacity retention and thermal properties, surface modifications (i.e., the application of coatings) have been made with many kinds of materials such as metal oxides,48−50 phosphates,51−53 fluorides,54,55 and conducting polymers.56,57 These coating layers can protect the surfaces of active materials from HF attack (i.e., HF scavengers), so that fluorinated protective films are found on the outermost surfaces of coating layers. Basically, these coating media are likely to bond with residual lithium compounds during the formation of the coating layer upon heating at moderate conditions. This may reduce the concentration of residual lithium compounds; for example, coating with Al2O3 can produce LiAlO2 on the outermost surface of the active materials because two Al atoms react with two residual Li atoms on the surface (Al2O3 + Li2O → 2LiAlO2). Recently, Jo et al.58 employed this concept using H3PO4 to reduce surface lithium compounds on Ni-rich cathode materials. In this case, three lithium atoms can be captured to form Li3PO4 during the formation of Li3PO4 coating layers: 3Li2O + 2H3PO4 → 2Li3PO4 + 3H2O, 3LiOH + H3PO4 → Li3PO4 + 3H2O, and 3Li2CO3 + H3PO4 → 2Li3PO4 + 3CO2 + 3H2O. Additionally, it is known that Li3PO4 is an ionic conductor (∼6 × 10−8 S cm−159,60); hence, its conductive properties are likely to improve the electrode performance. This treatment could dramatically reduce the residual lithium content of Li[Ni0.6Co0.2Mn0.2]O2 from 5220 to 1935 ppm. This full cell showed outstanding capacity retention after 1000 cycles, maintaining 95.6% of the first discharge capacity. These improvements were mainly attributed to the decrease in the residual lithium compounds on the surface of

Figure 2. Schematic illustration of chemistry on the surface for bare and lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 (reprinted from ref 58).

Mn, the delithiated Li1−δ[Ni0.5Mn0.5]O2 exhibited outstanding structural and thermal stabilities; the main exothermic reaction appears above 280 °C for Li0.25[Ni0.5Mn0.5]O2.42 Another material using the Li[Ni2+0.5Mn4+0.5]O2 framework is Li[Ni 2+1/3Co3+1/3Mn4+1/3]O 2.43 Because the added Co is stabilized as Co3+, there are no changes in the oxidation states of Ni and Mn in the compound. An additional advantage of this compound is the presence of Co3+, which increases the electric conductivity and decreases cation mixing in the Li layers to 3% (Li1−δ[Ni0.5Mn0.5]O2 typically shows mixing of 10%).44 This helps improve the capacity and rate capability. In consideration of the energy density and capacity, however, both Li[Ni0.5Mn0.5]O2 and Li[Ni1/3Co1/3Mn1/3]O2 (NCM) materials have limited capacities of 150−160 mAh g−1 in the voltage range of 2.5−4.3 V. These values are lower than what is observed in NCA materials (200 mAh g−1), although the presence of Mn4+ in the oxide lattice ensures structural and thermal stability. This motivated the development of Ni-rich NCM materials, including Li[Ni1−x−yCoxMny]O2. It is a general concept that increasing the Ni content is beneficial to increasing the capacity; unfortunately, this also leads to fast capacity fading upon cycling. Similar to NCA materials, delithiated Ni-rich cathode materials undergo a violent exothermic decomposition reaction that is accompanied by oxygen evolution from the oxide lattice in the temperature range of 150−300 °C. Another urgent issue that must be resolved is that both NCA and NCM Ni-rich materials are very sensitive to moisture and air. Excess amounts of lithium are always required to yield highly crystalline Ni-rich materials. As a result, unreacted residual lithium ingredients are inevitably formed on the surface 705

DOI: 10.1021/acsenergylett.7b00130 ACS Energy Lett. 2017, 2, 703−708

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ACS Energy Letters

Figure 3. Schematic drawings of (left) a core−shell (Gen 1), (centre) a Ni-rich core surrounded by a concentration gradient outer layer (Gen 2), and (right) a FCG lithium transition metal oxide particle (Gen 3) with the nickel concentration decreasing from the center toward the outer layer and the concentration of manganese increasing accordingly (reprinted from ref 67).

Li[Ni0.6Co0.2Mn0.2]O2 by forming the Li3PO4 coating layers from H3PO4. Additionally, the coating layer played additional functions such as absorbing the water of Li3PO4 (which produced less HF upon the decomposition of LiPF6 salt), scavenging HF (which reduced the level of HF in the electrolyte), and protecting the active material from HF attack during cycling. Further experiments must still be carried out to further reduce the residual lithium compounds on the surface of Ni-rich cathode materials (Ni > 80%). The above review explains that it is difficult to develop highenergy cathode materials that guarantee high capacity, retention, and acceptable thermal properties. Recently, Sun’s group proposed core−shell NCM particles to enhance the thermal stability of the composite without decreasing the energy density.61,61−67 The idea behind the proposed core− shell NCM particles is that the core of the particles is enriched with Ni, which is responsible for the high discharge capacity, while the shell is enriched with Mn to improve the chemical stability of the composite, as illustrated in Figure 3 (left).61 The thickness of the shell of the core−shell NCM particle should be 1−2 μm because LiNi0.5Mn0.5O2 has a poor rate capability; a thin shell can show reasonable performance at a high current density. Although the inclusion of Li[Ni0.5Mn0.5]O2 decreased the overall capacity, the cells could be operated for 500 cycles at a rate of 1 C with an excellent capacity retention of 98%. This outstanding performance was achieved by employing the Li[Ni0.5Mn0.5]O2 shell, which prevented Co dissolution from the structure via HF attack. In addition, the thermal stability of the composite was improved with the help of the thermally stable Li[Ni0.5Mn0.5]O2 shell, which suppressed the release of oxygen from the structure.61 However, it was found that the difference in the volume expansion between the Ni-rich core (9−10%) and the Mn-rich shell (2−3%) during Li deintercalation causes damage to the particles after long-term cycling.62,63 This causes degradation of the cathode materials. In order to overcome the volume expansion differences between the core and shell and achieve a higher capacity, a core−shell concentration gradient (CCG) was introduced (Figure 3 (center)).64 The composition from the bulk to the surface is changed from Li[Ni0.8Co0.1Mn0.1]O2 to Li[Ni0.46Co0.23Mn0.31]O2. The CCG composite shows a slightly lower initial capacity; however, 97% of the initial capacity remains after 500 cycles at 1 C, which is superior to the core composite without the concentration gradient shell. The decreasing amount of Ni on the surface suppressed side reactions with the electrolyte, ensuring long cyclability. In addition, it can improve the safety of the batteries.64

In order to further improve the composition of the CCG, a full concentration gradient (FCG), in which the concentration gradients of Ni, Co, and Mn were fully stretched through the particle, was introduced. The scheme of the FCG is illustrated in Figure 3 (right).65 The FCG composite demonstrated outstanding electrochemical performance maintaining 90% capacity retention after 1000 cycles, due to the needle-like nanosize primary particles. Further work was done using a two-sloped full concentration gradient (TSFCG) composite.66 The idea behind the TSFCG composite is to increase the Ni concentration near the core and the Mn concentration near the surface. The microscale primary particles of the TSFCG composite promote excellent electrochemical performance. After 1500 cycles at a current density of 1 C, the TSFCG cathode electrode retained 88% of its capacity. The excellent cyclability indicates that the TSFCG composite suppressed transition metal dissolution. The reason for the lower dissolution of metals comes from the lower surface area; TSFCG consists of rod-shaped particles. The low surface area also had a positive effect on the safety of the cathode electrode. The Mn-rich surface acted as a protective layer against HF attack. Since its commercialization, Li-ion batteries have dominated the market by fueling portable electronic devices and EVs. However, when it comes to EV applications, the driving distance per charge still needs to be prolonged; this is directly dependent on the battery energy density. In addition, considering recent fire-related accidents with EVs, the safety of the batteries must be improved. In order to overcome these issues, batteries should deliver high energy density and long cycle life. In addition, they should have good thermal stability in the charged state. All of these factors are dependent on the composite materials of the cathode. Nickel-rich layered oxides are one of the most promising cathode materials as an alternative to LiCoO2 cathodes, which provide high energy density for EV applications. Nickel-rich materials are appealing because they have high reversible capacity (200−220 mAh g−1), high operating voltage (∼3.8 V vs Li/Li+), and better chemical stability in the charged state. However, a high Ni content leads to poor capacity retention and thermal stability, despite the high capacity. In order to improve the performance of Ni-rich cathode composites, several approaches have been applied (as discussed above). Coating the surface of Ni-rich materials suppresses the dissolution of transition metals into the electrolyte and provides a HF scavenger to protect the surface of the cathode from HF attack. However, the thermal properties of these materials have not been discussed much. Another proposed solution to enhance the electrochemical 706

DOI: 10.1021/acsenergylett.7b00130 ACS Energy Lett. 2017, 2, 703−708

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ACS Energy Letters

(7) MacNeil, D. D.; Dahn, J. R. The Reactions of Li0.5CoO2 with Nonaqueous Solvents at Elevated Temperatures. J. Electrochem. Soc. 2002, 149, A912−A919. (8) Tukamoto, H.; West, A. R. Electronic Conductivity of LiCoO2 and Its Enhancement by Magnesium Doping. J. Electrochem. Soc. 1997, 144, 3164−3168. (9) Jang, Y.-I.; Huang, B.; Wang, H.; Sadoway, D. R.; Ceder, G.; Chiang, Y.-M.; Liu, H.; Tamura, H. LiAlyCo1‑yO2 (R3m) Intercalation Cathode for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1999, 146, 862−868. (10) Cho, J.; Kim, C.-S.; Yoo, S.-I. Improvement of Structural Stability of LiCoO2 Cathode during Electrochemical Cycling by SolGel Coating of SnO2. Electrochem. Solid-State Lett. 1999, 3, 362−365. (11) Ravet, N.; Chouinard, Y.; Magnan, J. F.; Besner, S.; Gauthier, M.; Armand, M. Electroactivity of Natural and Synthetic Triphylite. J. Power Sources 2001, 97−98, 503−507. (12) Koltypin, M.; Aurbach, D.; Nazar, L.; Ellis, B. On the Stability of LiFePO4 Olivine Cathodes under Various Conditions (Electrolyte Solutions, Temperatures). Electrochem. Solid-State Lett. 2007, 10, A40−A44. (13) Amine, K.; Liu, J.; Belharouak, I. High-temperature Storage and Cycling of C-LiFePO4/graphite Li-ion Cells. Electrochem. Commun. 2005, 7, 669−673. (14) Guerfi, A.; Ravet, N.; Charest, P.; Dontigny, M.; Petitclerc, M.; Gauthier, M.; Zaghib, K. Temperature Effects on LiFePO4 Cathode Performance. ECS Trans. 2006, 3, 3−17. (15) Chung, S.-Y.; Bloking, J. T.; Chiang, Y.-M. Electronically Conductive Phospho-olivines as Lithium Storage Electrodes. Nat. Mater. 2002, 1, 123−128. (16) Tarascon, J. M.; Guyomard, D. Li Metal-Free Rechargeable Batteries Based on Lil+xMn2O4 Cathodes (0< x< 1) and Carbon Anodes. J. Electrochem. Soc. 1991, 138, 2864−2868. (17) Yamada, A.; Tanaka, M. Jahn-Teller Structural Phase Transition around 280K in LiMn2O4. Mater. Res. Bull. 1995, 30, 715−721. (18) Thackeray, M. M. Spinel Electrodes for Lithium Batteries. J. Am. Ceram. Soc. 1999, 82, 3347−3354. (19) Hunter, J. C. Preparation of a New Crystal Form of Manganese Dioxide: A-MnO. J. Solid State Chem. 1981, 39, 142−147. (20) Ohzuku, T.; Takeda, S.; Iwanaga, M. Solid-state Redox Potentials for Li[Me1/2Mn3/2]O4 (Me: 3d-transition metal) having Spinel-framework Structures: A series of 5 V Materials/for Advanced Lithium-ion Batteries. J. Power Sources 1999, 81−82, 90−94. (21) Bang, H.-J.; Donepudi, V. S.; Prakash, J. Preparation and Characterization of Partially Substituted LiMyMn2‑yO4 (M = Ni, Co, Fe) Spinel Cathodes for Li-ion Batteries. Electrochim. Acta 2002, 48, 443−451. (22) Gnanaraj, J. S.; Pol, V. G.; Gedanken, A.; Aurbach, D. Improving the High-Temperature Performance of LiMn2O4 Spinel Electrodes by Coating the Active Mass with MgO via a Sonochemical Method. Electrochem. Commun. 2003, 5, 940−945. (23) Wu, H. M.; Belharouak, I.; Abouimrane, A.; Sun, Y.-K.; Amine, K. Surface Modification of LiNi0.5Mn1.5O4 by ZrP2O7 and ZrO2 for Lithium-ion Batteries. J. Power Sources 2010, 195, 2909−2913. (24) Amine, K.; Tukamoto, H.; Yasuda, H.; Fuiita, Y. A New ThreeVolt Spinel Li1+Mn1.5Ni0.5O4 for Secondary Lithium Batteries. J. Electrochem. Soc. 1996, 143, 1607−1613. (25) Zhong, Q.; Bonakclarpour, A.; Zhang, M.; Gao, Y.; Dahn, J. R. Synthesis and Electrochemistry of LiNixMn2‑xO4. J. Electrochem. Soc. 1997, 144, 205−213. (26) Lu, Z.; MacNeil, D. D.; Dahn, J. R. Layered Li[NixCo1‑xMnx]O2 Cathode Materials for Lithium-Ion Batteries. Electrochem. Solid-State Lett. 2001, 4, A200−A203. (27) Ohzuku, T.; Komori, H.; Nagayama, M.; Sawai, K.; Hirai, T. Electrochemical Characteristics of LiNiO2. Chem. Express 1991, 6, 161. (28) Dahn, J. R.; von Sacken, U.; Michal, C. A. Structure and Electrochemistry of Li1±yNiO2 and a New Li2NiO2 Phase with the Ni(OH)2 Structure. Solid State Ionics 1990, 44, 87−97.

properties of Ni-rich cathodes is to make a core−shell composite. The core has a Ni-rich composite, which is responsible for the high capacity, and the shell contains a Mn-rich composite to provide safety. The optimized TSFCG composite showed good electrochemical performance as well as desirable thermal properties. Further improvements in the electrochemical properties via surface modifications with inorganic oxides or organic compounds should also be considered in the future to overcome the problems related to residual lithium compounds. Due to the superior electrochemical performance and thermal properties of the TSFCG composite, we believe that this material has the potential to be a next-generation cathode material to power EVs.

Aishuak Konarov† Seung-Taek Myung*,† Yang-Kook Sun*,‡ †

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Department of Nano Technology and Advanced Materials Engineering & Sejong Battery Institute, Sejong University, Seoul 05006, South Korea ‡ Department of Energy Engineering, Hanyang University, Seoul, 04763, South Korea

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 82 2 3408 3454. Fax: 82 2 3408 4342 (S.-T.M.). *E-mail: [email protected]. Tel: 82 2 2220 0524. Fax: 82 2 2282 7329 (Y.-K.S.). ORCID

Yang-Kook Sun: 0000-0002-0117-0170 Notes

Views expressed in this Viewpoint are those of the author and not necessarily the views of the ACS. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2014R1A2A1A13050479) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20152000000650).

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DOI: 10.1021/acsenergylett.7b00130 ACS Energy Lett. 2017, 2, 703−708