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EXTREME FAST CHARGING FOR LITHIUM ION BATTERIES: STRUCTURAL ANALYSIS OF ELECTRODES AND SOLVENT FORMULATION OF ELECTROLYTES
Fossil fuel has dominated the global energy market for centuries, and the world is undergoing a great energy revolution from fossil fuel energy to renewable energies, given the concerns on global warming and extreme weather caused by the emission of carbon dioxide. Lithium ion batteries (LIBs) play an irreplaceable role in this incredible energy transition from fossil energy to renewable energy, given their importance in energy storage for electricity grids and promoting the mass adoption of battery electric vehicles (BEVs). Extreme fast charging (XFC) of LIBs, aiming to shorten the charging time to 15 minutes, will significantly improve their adoption in both the EV market and grid energy storage. However, XFC has been significantly hindered by the relatively sluggish Li+ transport within LIBs.
Herein, effects caused by increasing charging rates (from 1C, 4C to 6C) on LiNi0.6Mn0.2Co0.2O2 (NMC622) || graphite cell were systematically probed via various characterization methods. From electrochemical test on their rate/long term cycling performance, the significant decrease in available capacity under high charging rates was verified. Structural evolutions of cycled NMC622 cathode and graphite anode were further probed via ex-situ powder diffraction, and it was found that lattice parameters a and c of NMC622 experience irreversible evolution due to loss of active Li+ within NMC622; no structural evolution was found for the graphite anode, even after 200 cycles under 6C (10 minutes) high charging rates. The aging behavior of liquid electrolyte was further analyzed via inductively coupled plasma-optical emission spectrometry (ICP-OES) and gas chromatography-mass spectrometry (GC-MS), increased Li+ concentration under higher charging rates and show-up of diethyl carbonate (DEC) and dimethyl carbonate (DMC) caused by transesterification both suggest faster aging/degradation of liquid electrolyte under higher charging rates.
Given the structural evolution of NMC622 caused by irreversible Li+ loss after long term cycling, the structural evolution of both NMC622 cathode and lithiated graphite anode were further studied via operando neutron diffraction on customized LiNi0.6Mn0.2Co0.2O2 (NMC622) || graphite cell. Via a quantitative analysis of collected Bragg peaks for NMC622 and lithiated graphite anode, we found the rate independent structural evolution of NMC622: its lattice parameters a and c are mainly determined by Li+ contents within it (x within LixNi0.6Mn0.2Co0.2O2) and follow the same evolution during the deintercalation process, from slowest 0.27 C charging to the fastest 4.4 C charging. For graphite intercalated compounds (GICs) formed during Li+ intercalating into graphite, the sequential phase transition from pure graphite → stage III (LiC30) → stage II (LiC12) → stage I (LiC6) phase under 0.27 C charging is consistent with previous studies. This sequential phase transition is generally maintained under increasing charging rates, and the co-existence of LiC12 phase and LiC6 was found for lithiated graphite under 4.4 C charging, mainly due to the large inhomogeneity under these high charging rates. Meanwhile, for the stage II (LiC12) → stage I (LiC6) transition, which contributes half the specific capacity for the graphite anode, quantitative analysis via Johnson-Mehl-Avrami-Kolmogorov (JMAK) model suggests it to be a diffusion-controlled, one-dimensional transition, with decreasing nucleation kinetics under increasing charging rates.
Based on the LiC12 → LiC6 transition process, strategies to improve the Li+ transport properties were further utilized. Various cosolvents with smaller viscosity, from dimethyl carbonate (DMC), ethyl acetate (EA), methyl acetate (MA) to ethyl formate (EF), were further tested by replacing 20% (weight percent) ethyl methyl carbonate (EMC) of typical 1.2 M LiPF6 salt solvated in ethylene carbonate (EC)/EMC solvents (with a weight ratio of 30:70). From the measurement of their ion conductivity, the introduction of these cosolvents indeed enhanced the Li+ transport properties. This was further verified by improved rate performance from 2C, 3C to 4C charging for liquid electrolytes using these cosolvents. Both X-ray absorption spectroscopy (XAS) and X-ray powder diffraction (XRD) indicated the increase of Ni valence state and structural evolution of NMC622, all resulting from the irreversible loss of active Li+ within the NMC622 cathode. From long term cycling performance and further analysis of interfaces formed between electrode and anode, the best performance of electrolyte using DMC cosolvent was attributed to the most stable solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) formed during the cycling.