High-Performance Li-Ion Batteries Using Nickel-Rich Lithium Nickel

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High-performance Li-ion Batteries using Nickel-rich Lithium Nickel Cobalt Aluminium Oxide-Nanocarbon Core-Shell Cathode: In operando X-ray diffraction SELVAMANI VADIVEL, Nutthaphon Phattharasupakun, Juthaporn Wutthiprom, Salatan Duangdangchote, and Montree Sawangphruk ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06553 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 2, 2019

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ACS Applied Materials & Interfaces

High-performance Li-ion Batteries using Nickel-rich Lithium Nickel Cobalt Aluminium OxideNanocarbon Core-Shell Cathode: In operando X-ray diffraction Selvamani Vadivel,†,‡ Nutthaphon Phattharasupakun,†,‡ Juthaporn Wutthiprom,†,‡ Salatan duangdangchote,†,‡ and Montree Sawangphruk,*,†,‡

†Centre

of Excellence for Energy Storage Technology (CEST), Vidyasirimedhi Institute

of Science and Technology, Rayong 21210, Thailand.

‡Department

of Chemical and Biomolecular Engineering, Vidyasirimedhi Institute of

Science and Technology, Rayong 21210, Thailand.

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ABSTRACT: Nickel-rich layered, mixed lithium transition-metal oxides have pursued as a propitious cathode material for future-generation lithium-ion batteries due to its high energy density and low cost. Nevertheless, acute side reactions between the Ni4+ and the carbonate electrolyte leading to poor cycling as well as rate performance, which limits large-scale applications. Here, core-shell like an NCA-carbon composite synthesized by a solvent-free mechano-fusion method is reported to solve this issue. Such a core-shell structure exhibits a splendid rate as well as stable cycling when compared to physically blended NCA. In operando X-Ray diffraction studies impart that both the material experiences anisotropic structural change, i.e., stacking c-axis undergoes gradual expansion followed by an abrupt shrinkage; meanwhile, a-axis contracts during the charging process and vice versa. Interestingly, the core-shell material displays a significantly high reversible capacity of 91 % in the formation cycle at 0.1 C and retention


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of 84 % at 0.5 C after 250 cycles, whereas the pristine NCA retains 71 %. The robust mechanical force assisted dry coating obtained by the mechano-fusion method shows improved electrochemical performance and demonstrates its practical feasibility.

KEYWORDS: Mechanofusion, NCA-Nanocarbon core-shell, In operando XRD, Ni-rich cathode, Anisotropic lattice change

INTRODUCTION Although the current lithium-ion technology is commercially successful, relentless researches are still focussed on enhancing the performance especially for high-energy applications like electric vehicles (EV), hybrid and plug-in electric vehicles (HEV, P-HEV) and grid storage. Among various cathode materials, the layered LiCoO2 is widely studied in commercial lithium-ion batteries. However, high cost, low practical capacity, scarcity, moderate cycling performance, and toxicity of Co limiting its usage for high-energy applications. Lately, nickel-rich layered, mixed transition metal oxides get much attention


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in the next-generation lithium-ion system owing to their high reversible specific capacity with low cost, and environmentally benign. Nevertheless, it is difficult to be synthesized with an accurate stoichiometry of LiNiO2 due to the comparable ionic radii of Ni2+ and Li+ leading to cationic disorder, which further ensures the formation of electrochemically inactive NiO phase.1,2 During the charging process, the Ni2+ existing at the lithium slab can be oxidized to Ni3+, reducing the interlayer spacing and thereby impeding the lithium ion diffusion path. Besides, the material is highly unstable at the charged or high valence state (Ni4+) and undergoes thermal runaway in electrolyte combined with an oxygen release. The partial substitution of d-block elements (Mn, Co, Al, Mg) in the place of the nickel site is an effective way to mitigate these detrimental cationic disorders and offstoichiometry.3 Current state-of-the-art cathodes are mainly focused on Ni-rich layered ternary metal oxides, especially Co, Al co-doping in a small amount (LiNi0.8Co0.15Al0.05O2, NCA) where the synergistic effect of the alloy helps to ameliorate the electrochemical properties of LiNiO2.4 Furthermore, electrochemically inactive Al not only supports to retain the layered structure but also improves thermal stability. While the slight


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compression occurs due to Co, at the interlayer tending to reduce the Li/Ni cation mixing.5–7 Although NCA has been assured as a propitious alternative electrode material for heavy-duty applications such as electric vehicles (Tesla); the problem remains still unsolved. NCA is facing severe capacity decay and poor cycling stability due to the conversion of rock-salt-like inactive NiO domains with the expense of reactive lattice oxygen and thermal runaway, Li2CO3 by-products, micro-cracks at the grain boundaries and the dissolution of Ni4+/Co3+ into the electrolyte.8 Moreover, the poor ionic conductivity and easy phase transformation (from the interior to surface) with temperature, from native layered (R-3m) structure to disordered spinel (Fd-3m) and rock-salt (Fm-3m) of NCA material also be a reason for its inferior electrochemical performance.9 Extensive research has sincerely devoted to overcoming these drawbacks and understanding the relative phase transformation during the redox reactions. Cation/anion doping and surface coating with metal oxides/phosphates/fluorides are the effective strategies to improve the interfacial stability, cyclability as well as rate performance.10 Although the ion doping method exhibits improved electrochemical performance, it is ineffective to prevent the parasitic side reactions at high voltage, reducing its original


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specific capacity due to the charge compensation. Likewise, the surface coating efficiently obstructs the contact between the electrolyte and active material but provides additional resistance due to poor electronic conductivity. Recently, conductive carbon coating gains much attention owing to low cost, improved conductivity as well as stability and secure electron transfer.11 So far, wet chemical and atomic layer deposition (ALD) methods are widely applied for surface coatings. However, the former process requires additional posttreatment, and the latter is difficult to scale-up as well as limited by choice of the chemical precursors. Lately, researchers are interested in studying the structural changes of electrode materials through phase transition during lithium extraction and insertion in a highly reactive electrolyte medium. Notably, significant variation in the lattice strain reflected in the unit cell is as a result of bond length change during the redox reaction of the transition metal.12 The in-operando analysis is more advantages than traditional ex-situ analysis because it can overcome many limitations of ex-situ measurements including surface contamination, and challenging to maintain the time interval for each measurement as well as unable to reuse the same electrode for all the measurement.13 Hitherto, most of


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the in situ studies are mainly focused on the formation cycle (the first cycle), and it is wise to study the subsequent cycles for complete understanding.14–16 Herein, a facile solvent-free and economically feasible mechano-fusion process were employed to coat the conductive nanocarbon (< 50 nm) as a surface modifier for a particle in micron. In this dry coating process, the nanoscale-sized guest particles are firmly embedded over the micron-sized host through strong mechanical forces. Although the fusion process is simple, the material undergoes a complex mechanism inside the rotating chamber.17 During the mechanothermal-fusion, the particles experience strong shear and compressive forces while it passes between the rotating chamber wall and fixed presshead of a rotor. Further, the fixed scrubber blade helps to re-disperse the centrifugally adhered particles on the wall of the chamber and repeat the task over numerous times. The high-speed operation (~ 5000 rpm) and the constant mechanical force with threedimensional dispersion, ensure uniform coating over the host particle without destroying the morphology. These coatings not only facilitate fast electron transfer but also acts as a buffer medium to prevent the micro-crack formation and direct electrolyte contact, thereby exhibiting reasonable rate and long-term stability. Furthermore, the complete


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structural change and phase evolution were also investigated during cycling using in

operando XRD experiment, which clearly shows the irreversible two-phase transition at the initial, after that a reversible solid solution reaction in the activation cycle. The anisotropic unit cell parameters (stacking c-axis and the a-axis) variation upon the galvanostatic extraction/insertion of lithium were also discussed in detail.

RESULTS AND DISCUSSION

Physicochemical properties. Figure 1 depicts the FE-SEM morphologies of conductive nanocarbon (Super P), pristine NCA and physically blended NCA with carbon (PNCA) and mechano-fused NCA core-shell (NCACS) like carbon composite. Super P (Figure 1a) shows interconnected carbon nanospheres (

High-Performance Li-Ion Batteries Using Nickel-Rich Lithium Nickel

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