Lithium metal deposition during overcharge in practical lithium ion cells composed of a lithium metal oxide (LiCoO 2 ) positive electrode coated on Al foil, carbon (synthesized graphite and hard carbon) negative electrodes coated on Cu foil, polypropylene separator, and liquid electrolyte was observed using in situ solid state 7 Li nuclear magnetic resonance (NMR) measurements with an original probe featuring a flattened solenoid coil. Li insertion and extraction in carbon electrodes were monitored during charge and discharge and the intensities of certain peaks were found to be proportional to the cell capacity change. The deposition of metallic Li commenced after the cell voltage exceeded the nominal value and almost saturated after 160% of charge at a low current rate. The measurements showed that the deposition of metallic Li was much easier on graphite compared to hard carbon. The metallic Li deposited on hard carbon was almost completely discharged, whereas that on graphite remained after discharging to 2.5 V.

(Abstract)Lithium ion cells comprising actual components of positive electrodes (LiCoO2, LiNixCoyAlz, and LiMn2O4) and negative electrodes (graphite and hard carbon) were assembled for in situ 7 Li nuclear magnetic resonance (NMR) experiments. The 7 Li NMR measurements of the cells revealed a "relaxation effect" after overcharging: a decrease of the signal assigned to Li metal deposited on the negative electrode surface by overcharging.The reduction of the Li metal signal was inversely proportional to the increase of the signal of lithium stored in carbon. Therefore, the effect was ascribed to absorption of deposited

The capacity fading of lithium ion batteries (LIBs) is investigated, using in situ 7 Li solid-state nuclear magnetic resonance (NMR). LIB cells consisting of graphite or hard carbon and LiCoO 2 are used and cycled under two different temperatures (10 and 25 • C) and charge current rates (0.5 and 1 C). The cell capacity and the amount of Li stored in carbon are measured with a battery cycler and in situ 7 Li solid-state NMR measurements before the beginning of the test and after every 100 test cycles. The in situ 7 Li solid state NMRs provide sufficient information throughout the cycle tests to characterize all test conditions. The cell capacities are analyzed in terms of the 7 Li NMR peak intensity, attributed to Li stored in carbon. This intensity shows affine proportionality with the cell capacity for every evaluation, with an additive constant that decreases with the increase in cell capacity fading. This may be related to the loss of Li storage caused by voltage "slippage. Lithium-ion batteries (LIBs) have been widely used in portable electronic devices and as power sources for mobile machines such as electric vehicles, hybrid electric vehicles, e-bikes, e-scooters, and electric wheelchairs. To ensure the sustainability and endurance of these devices, it is necessary to understand battery degradation (in both capacity and power) due to daily use.1 LIBs are charged and discharged by Li insertion without chemical reactions, and the degradation of electrode materials is consequently small. LIBs are therefore widely considered to be excellent batteries, with long cycle lives. However, capacity fading occurs when LIBs are used outside moderate temperature ranges, at high current, with high voltage storage, etc. The capacity balance between positive and negative electrodes is lost (called "slippage") because of charge and discharge operations with low accuracy controllers or parasitic reactions during operation and storage.2-9 Slippage causes gradual capacity fading, because cell capacity is determined by the capacity balance between the positive and negative electrodes.Previous studies have reported how and when slippage induces capacity fading, but not how slippage occurs and what it is exactly. To fully understand capacity fading and slippage, nondestructive measurements are needed, because capacity fading due to slippage will only be properly observed if the cell internal conditions are fully maintained.To examine the state inside the battery without destroying it, we developed an in situ 7 Li solid-state nuclear magnetic resonance (NMR) measurement method, using full-cell test equipment consisting of actual positive and negative electrodes. Using this method, we studied Li insertion/extraction in carbon with charge/discharge operations, Li dendrite growth during overcharges, and the properties of dendritic Li after being formed. 10,11 This study investigated the change in NMR peak intensity of the Li stored in the cycle test, to elucidate cell capacity fading in terms of the amount of Li storage at the select...

The capacity fade of lithium ion battery (LIB) was investigated using 7Li solid-state nuclear magnetic resonance (NMR). The LIB cells composed of graphite (Gr) or hard carbon (HC) and LiCoO2 (LCO) were used. The peak intensity for stored Li in carbon (Gr and HC) showed good proportionality to the cell capacity. The proportional constants for every after 100 cycles did not change largely, while the section of the peak intensity vs. cell capacity plot changed with cycle. This change in the section explained well the cell capacity fade, showing that “slippage” was related to the capacity loss in lower stages due to voltage profile of carbons. The peak analysis also found “captured” Li loss as another cause of capacity fade. The NMR measurement also found new storage peak and change in solid electrolyte interface (SEI) related peak after long cycle test.

Lithium metal deposition during overcharge in a full cell composed of lithium metal oxide (LiCoO 2 ) positive electrodes and carbon negative electrodes (a synthesized graphite, a natural graphite and two types of hard carbon) was observed by in situ 7 Li solid state NMR (nuclear magnetic resonance). Peak intensities corresponding to the GIC (Graphite Intercalation Compound, charged state) and dendritic Li (deposited metallic Li) were monitored at each 20% state of charge (SOC) during discharge (and charge) and each 17 minutes (or 1000 scans) after the cell capacity reached the expected overcharge. The GIC peak increased with charge and decreased with discharge during the normal voltage of operation. The Li metal deposition occurred after the cell was charged above its normal operational voltage (4.2 V) though intercalation into the carbon still simultaneously occured, suggesting that metallic Li can be formed when the negative electrode reaches a voltage below 0 V vs. Li. We also found that the dendritic Li is not inert but active to be oxidized again and intercalated into the carbon within a few hours. The dendritic Li formed on hard carbon was completely discharged, while that deposited on graphite was not, indicating that the electrochemical property of the dendritic Li varies with the nature of the negative electrode. ECS Transactions, 62 (1) 159-187 (2014) 159 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 169.230.243.252 Downloaded on 2014-12-14 to IP