In Situ Solid State 7li NMR Observation of Lithium Metal Deposition during Overcharge of Lithium Ion Battery

Understanding the reaction inside sealed lithium ion battery (LIB) is a key to handle it safe and life well, and to improve its performance. In situ measurement is necessary to achieve it, because LIB has a quasi equilibrium state in its system which is no more existing if it is disassembled. In situ NMR is one of the most suitable technique which can detect the change in lithium. So far, excellent studies have been done using modified electrodes and electrolyte to obtain a good NMR signal. [1,2] But, the LIB performance and reaction may change depending on the electrode density, electrolyte, packaging, and etc. This paper reports in situ solid NMR measurement using small size cell composed of high density coated electrodes and ordinal liquid electrolyte with heat sealed pouch cell having Al coat layer.[3]

Lithium deposition and Lithium ion behaviors of Lithium battery composed of cathode selected from LCO, NMC, NCA, LMO and graphite anode during charge-discharge and overcharge were analyzed by in-situ ⁷Li solid state NMR. The cell used in this study was made using typical energy density coated electrode, ordinal 25 mm polypropylene separator, 1M LiPF6 EC/EMC (3/7 vol%) liquid organic electrolyte and laminate film containing Al layer. The average cell capacity is 4.3 mAh with 1×1.5 cm electrode size. In situ NMR measurements were carried out with an originally prepared flattened coil detector and home build NMR equipment with 7.0T magnet (resonance frequency 116.4 MHz for ⁷Li). A single pulse sequence was applied with 6 ms pulse length. ⁷Li chemical shifts were referenced to a signal of saturated LiCl solution (0 ppm).

The peak corresponding to metallic Lithium was observed after 1 mAh overcharge from SOC 100% for LCO, NMC and NCA cell. The intensity of Li metallic peak increased with overcharge capacity in these cells, while the metallic peak was not appeared in LMO cell due to Li content in its. The peak after overcharge in LCO cell was the strongest in these cells and did not disappeared after one night, but the peak in NCA gone after one night. This suggests that the stability of Lithium dendrite depends on the cell chemistry.

The peak belonging to Li ion in electrolyte split into two peaks which may due to the surface interaction with cathode electrode. The peak ratio among these two in LCO cell decreased with discharge, indicating the peak is influenced by the state of cathode structure, voltage or relaxation of ion flow.

[1] Michel Letellier, Frederic Chevallier, Christian Clinard, Elzbieta frackowiak, Jean-Noel Rouzaud, and Francois Beguin, J. Chem. Phys., 118, p.6038 (2003).

[2] Nicole M. Trease, Lina Zhou, Hee Jung Chang, Ben Yunxu Chang, Clare P. Grey, Sol. Sta. Nuc. Mag. Res., 42, p.62 (2012).

[3] Arai Juichi, Okada Yumika, Takeda Kazuyuki, Gotoh Kazuma, Izuka Masato and Ishida Hiroyuki, The 7^th Asian Conference on Electrochemical Power Sources, Abstract 1P-19, p.88 (Osaka)2013.11.24-27