Article pubs.acs.org/JPCC
Effects of Antisite Defects on Li Diffusion in LiFePO4 Revealed by Li Isotope Exchange Hao Liu,†,∥,§ Min-Ju Choe,‡,∥ Raul A. Enrique,‡ Bernardo Orvañanos,‡ Lina Zhou,† Tao Liu,† Katsuyo Thornton,*,‡ and Clare P. Grey*,† †
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, Cambridgeshire CB2 1EW, United Kingdom Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States
‡
S Supporting Information *
ABSTRACT: Li−Fe antisite defects are commonly found in LiFePO4 particles and can impede or block Li diffusion in the single-file Li diffusion channels. However, due to their low concentration (∼1%), the effect of antisite defects on Li diffusion has only been systematically investigated by theoretical approaches. In this work, the exchange between Li in solid LiFePO4 (92.5% enriched with 6Li) and Li in the liquid Li electrolyte solution (containing natural abundance Li, 7.6% 6 Li and 92.4% 7Li) was measured as a function of time by both ex situ and in situ solid-state nuclear magnetic resonance experiments. The experimental data reveal that the time dependence of the isotope exchange cannot be modeled by a simple single-file diffusion process and that defects must play a role in the mobility of ions in the LiFePO4 particles. By performing kinetic Monte Carlo simulations that explicitly consider antisite defects, which allow Li to cross over between adjacent channels, we show that the observed tracer exchange behavior can be explained by the presence of channels with paired Li−Fe antisite defects. The simulations suggest that Li diffusion across the antisite is slow (10−16 cm2 s−1) and that the presence of antisite defects is widespread in the LiFePO4 particles we examined, where ∼80% channels are affected by such defects.
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methods11−13 probe the microscopic Li hopping event, which is dominated by the Li hopping process in the diffusion channel and is not sensitive to Li hopping across antisite defects. Electrochemical methods, such as cyclic voltammetry,14,15 galvanostatic/potentiostatic titration techniques,15−18 and electrochemical impedance spectroscopy,15 are based on diffusion models derived from Fick’s laws, which do not reproduce the time evolution expected in single-file diffusion, nor can they readily account for additional complex mechanisms such as the effects of antisite defects. Tracer exchange measurement provides an alternative method for diffusivity measurement and has been used to study molecular diffusion in zeolites.19−21 For 1D diffusion systems such as LiFePO4, the tracer exchange process can be very sensitive to even a low concentration of defects that block or impede the 1D diffusion. For LiFePO4, the tracer exchange can be quantified by the fraction of Li in LiFePO4 that has undergone exchange with the Li in the electrolyte solution. This underlies the microscopic mechanism of the exchange current. If Li in LiFePO4 and the external reservoir is initially labeled with 6Li and 7Li, respectively, all 6Li in LiFePO4 will eventually be replaced with 7Li, provided the reservoir is infinite and no Li in
INTRODUCTION Solid-state Li ion diffusion plays an important role in controlling the rate capability of any Li ion battery electrode material. Li ion diffusion can occur in different dimensionalities depending on the crystal structure of the specific compound: the isolated Li ion channels in olivine-type LiFePO41,2 lead to one-dimensional (1D) diffusion, layered LiMO2 (M = Ni, Co, Mn)3,4 allows two-dimensional (2D) diffusion between the MO6 octahedral layers, and the cubic spinel LiMn2O45,6 enables isotropic three-dimensional (3D) diffusion. Both 2D and 3D diffusions are conducive to fast Li diffusion and are more tolerant to defects than 1D diffusion. In 1D diffusion, where only single-file passage of the Li ions in the diffusion channel is possible, the presence of defects in the diffusion path will greatly impede or even block the Li diffusion. For instance, the effect of Li−Fe antisite defects on the Li diffusion barrier has been theoretically demonstrated by Malik et al.7 for LiFePO4. When antisite defects are present, they force Li to hop across channels, which is predicted to have a much higher energy barrier than Li hopping along the channel.8,9 Despite much theoretical investigation, the cross-channel Li diffusivity via a Li−Fe antisite has not been probed by experiment due to the small concentration (a few or less than 1%)7,10 of the antisite defects in the sample. A variety of experimental techniques are available for measuring the Li ion diffusion processes. Muon spin relaxation © 2017 American Chemical Society
Received: March 24, 2017 Revised: May 13, 2017 Published: May 16, 2017 12025
DOI: 10.1021/acs.jpcc.7b02819 J. Phys. Chem. C 2017, 121, 12025−12036
Article
The Journal of Physical Chemistry C
For the synthesis carried out with PEG solvent, 0.00225 mol of H3PO4 (85 wt % aqueous solution, Sigma-Aldrich) was mixed with 2.25 mL of deionized water and 9 mL of PEG. A 0.00675 mol sample of 6Li-enriched LiOH·H2O (Cambridge Isotope Laboratories, 96% 6Li) and 0.00225 mol of FeSO4· 7H2O (Fisher Scientific) were dissolved in 6.75 and 4.5 mL of deionized water, respectively. The FeSO4 solution was added dropwise to the H3PO4 solution under mechanical stirring, resulting in the formation of white precipitates. The LiOH solution was subsequently introduced dropwise into the mixture, and the precipitates turned a light green color. The mixture was stirred for an additional 5 min before being transferred to a 45 mL Teflon sleeve. The mixture in the Teflon sleeve was purged with Ar for 2 min, sealed in a Parr reactor, and kept at 180 °C for 24 h. The product was collected by centrifuge, washed with water and ethanol, and dried in a vacuum oven at 100 °C overnight. To prepare samples of different overall Li stoichiometries (and thus occupancies, η, of Li in the 1D channels), the sample synthesized in EG was partially delithiated by Br2 via the following reaction:24
LiFePO4 is trapped between the Li−Fe antisite defects. If the external reservoir contains a finite amount of 7Li, the fractions of 7Li in LiFePO4 and the reservoir will become equal at equilibrium. The fraction of 7Li in LiFePO4 as a function of time will only be dictated by the Li hopping rate, Li occupancy in the channel, the channel length, and the rate of Li exchange between the channel and the reservoir. When the Li diffusion along the channel is impeded by the antisite defects, the time evolution of 7Li in LiFePO4 will deviate from that predicted for an ideal defect-free channel and thus would allow us to probe the cross-channel Li diffusivity. Previous calculations have estimated the equilibrium Li−Fe antisite defect concentration in the range between 0.1% and 0.5%,7 which corresponds to one defect per 300 or 60 nm channel, respectively (assuming a random defect distribution and 0.3 nm as the distance between two adjacent Li sites). Therefore, even for LiFePO4 particles synthesized under optimal conditions with short Li diffusion channels (
