Using the Charge-Discharge Cycling of Positive Electrode Symmetric Cells to Find Electrolyte/Electrode Combinations with Minimum Reactivity

Abstract

The effects of solvents, salts, electrolyte additives and surface coatings on LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442) or LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) have been probed using positive electrode Li-ion symmetric cells coupled with dV/dQ analysis. A robust symmetric cell design is presented which prevents hardware corrosion for over at least 800 hours of testing to 4.5 V vs. Li/Li + at 40 • C. Positive electrode symmetric cells using uncoated positive electrode materials and 1M LiPF 6 EC:EMC 3:7 or 1M LiPF 6 EMC electrolyte rapidly developed high impedance and showed poor capacity retention. However, if 1% pyridine boron trifluoride (PBF) was added to these electrolytes, cell performance was dramatically improved. Replacing LiPF 6 by LiBF 4 in the electrolytes above, with or without PBF, yielded positive electrode symmetric cells with good capacity retention. Two types of surface coatings were explored on NMC622 positive electrodes. Cells using surface-coated positive electrodes demonstrated better capacity retention for all electrolytes compared to cells without surface coatings. This work can be used as a guide by those attempting to find electrolyte/electrode pairs suitable for use in NMC/graphite cells that can operate with long lifetime to 4. 1-4 The energy density of NMC/graphite cells can be easily improved if the cells can be charged to higher potential, for example to 4.4 or 4.5 V at which point the positive electrode is at about 4.48 or 4.58 V vs. Li/Li + . However, charging NMC/graphite cells to such potentials normally leads to impedance growth and a reduction in cycle and calendar life. 5,6 Increasing the charging cutoff voltage to 4.40 V can cause some problems: 1) Parasitic reactions like electrolyte oxidation can create harmful products and damage the surface of the electrodes. 7,8 In addition, in some cases, thick SEI layers on the positive electrode can be created.2) The layered surface structure of NMC (R3m) can change into a spinel-like framework (Fd3m) and then to the disordered rocksalt structure (Fm3m). 9,10 This is thought to be one cause of impedance growth at the positive electrode.3) The unit cell volume of most NMC materials begins to contract rapidly, by about 4%, when the potential is raised above 4.4 V vs Li/Li + as the lithium content is decreased below about z = 0.25 in Li x Ni 1-y-z Mn y Co z O 2 . This could induce a detachment between binder and active particles along with cracks forming inside the secondary particles. 11,12A number of strategies have been used to protect the positive electrode surface from parasitic reactions. These include the use of electrolyte additives, different salts, different solvents and electrode material coatings. Some of these strategies will be used in the work here. For example, Petibon et al. and Ma et al. 13,14 showed that ethylene carbonate (EC)-free electrolytes, like 1M LiPF 6 in 95% ethyl methyl carbonate with 5% fluoroethylene carbonate was very effective in allowing NMC/graphite cells to operate to 4.4 V. EC-free electrolytes will be expl...

Review of Boron Compounds in Lithium Batteries: Electrolyte, Anode, Cathode, Separator, and Battery Thermal Management System (BTMS)

“…Similar effectiveness was also found in a pouch cell experiment on NMC532 and NMC622 [20]. A combination of using PBF as additives and LiBF4 as electrolytes for NMC442 provided incremental benefits compared to using LiBF4 as electrolytes [15]. Figure 2 shows the molecular structure of the abovementioned boron compounds.…”

“…Around the time of the first report of LiBOB, LiBF4 was found to have lower conductivity but better low-temperature performance than LiPF6 [14]. Later in LIB development, LiBF4 salt in carbonate electrolytes was tested as a direct replacement for LiPF6, which improved capacity retention in NMC622 and NMC442 systems due to a more stable B-F bond compared to the P-F bond [15][16][17]. When only used as an additive to LiPF6, 1 to 2 wt% of LiBF4 significantly enhanced capacity retention in the NMC532 system, suggesting the likely mechanism that LiBF4 contributed to the formation of SEI on both electrodes [18].…”

Deciphering Interfacial Reactions via Optical Sensing to Tune the Interphase Chemistry for Optimized Na‐Ion Electrolyte Formulation

“…To interrogate the cross‐talking between electrodes, we followed Jeff Dahn's approach of symmetric cells cycling. [ 45–47 ] Cells using desodiated Na≈ 1 V 2 (PO 4 ) 2 F 3 | sodiated Na≈ 3 V 2 (PO 4 ) 2 F 3 and desodiated HC | sodiated HC, respectively were assembled with electrodes prepared and recovered according to the protocol described in the Experimental Section. The representative cycling curves and individual electrode curves calculated with d V /d Q fitting software [ 41 ] are presented in Figure S11 and Note S1, Supporting Information.…”

“…To interrogate the cross-talking between electrodes, we followed Jeff Dahn's approach of symmetric cells cycling [45][46][47] . Cells using desodiated Na 1 V 2 (PO 4 ) 2 F 3 | sodiated Na~3V 2 (PO 4 ) 2 F 3 and desodiated hard carbon | sodiated hard carbon, respectively were assembled with electrodes prepared and recovered according to the protocol described in experimental section.…”

Temperature Dependence of Electrochemical Degradation in LiNi1/3Mn1/3Co1/3O2/Li4Ti5O12 Cells

“…Similar results have previously been shown for symmetric LTO–LTO cells with minor capacity fading at 30 °C but significant fading at 60 °C, which was attributed to electrolyte reduction . Rapid capacity fading in NMC–NMC cells has previously been observed at 30 °C and also in NMC442–NMC442 cells cycled at 40 °C, attributed to side‐reactions rather than mechanical degradation . The fading in the LTO–LTO cell is likely related to gas formation, which has been shown to be prominent at elevated temperatures .…”