Slow Stabilization of Si-Li Alloys Formed during Charge and Discharge of a Si-C Mixed Electrode Studied by In Situ Solid-State⁷Li Nuclear Magnetic Resonance Spectroscopy

Abstract

The charge-discharge behavior of a Si-C mixed electrode (Si:graphite = 20:80 or 30:70 (wt%)) was investigated using in situ solidstate 7 Li nuclear magnetic resonance (NMR) spectroscopy. The spectra revealed the formation of Li-Si alloys and the intercalation of Li into graphite during the charge process and the corresponding reverse process (Li extraction) during discharge. Li was mainly stored as a Li-Si alloy (Li 15 Si 4 or Li 15+δ Si 4 ) at high SOC (state of charge) values (above 80% SOC, low cell voltage region) and could be released from the alloy. The cell resistance measured by electrochemical impedance spectroscopy (EIS) increased steeply at high SOC values, revealing that structural changes in the Li-Si alloy may influence the electrode resistance. Lithium-ion batteries (LIBs) have been widely used in consumer electric appliances and gradually introduced in electric vehicles, such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and e-bikes.1,2 These power applications still require the realization of higher energy and power densities to extend the available driving distance, improve insufficient power assistance, and overcome battery regeneration issues. With the aim of improving the energy density of LIBs, a wide variety of active materials with high specific energy densities have been studied for negative and positive electrodes, such as Li-rich LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC), LiNi 1/2 Mn 3/2 O 4 , and FeF 3 for positive electrodes, and Si, Sn, and FeO for negative ones.3-7 Among these materials, Si has recently been used in Si-C mixed negative electrodes for commercial portable batteries, which have become the leader of 18650-cells owing to their high energy density, although the cell performance still needs to be improved.One of the challenges when using Si negative electrodes is managing the formation of Li-Si alloys and their variation with the charging progress (lithiation), as high Li contents induce volume expansion of the alloys, which then become reactive toward the electrolyte. 8,9 Understanding the formation of alloys during electrochemical processes is necessary in order to control the reactions and improve the cell performance. Key et al. examined the lithiation of pure Si in an electrochemical process using ex situ and in situ Li nuclear magnetic resonance (NMR) spectroscopy in combination with other characterization methods.10,11 The NMR technique was applied to understand the electrochemical reaction of Li with Si.12-15 The changes in cell resistance during alloying of Si with Li have also been studied by electrochemical impedance spectroscopy (EIS). [16][17][18] This study investigated the lithiation of Si in a more practical coated Si-C mixed electrode using in situ solid-state 7 Li NMR spectroscopy and EIS to understand the lithiation reactions and stability of Si. ExperimentalCell preparation and electrochemical measurements.-Mixtures of Si and graphite (Si:graphite = 20:80 or 30:70 wt%) were manually blended with a polyimide binder dissolved in N-methylpy...

Mechanisms for overcharging of carbon electrodes in lithium-ion/sodium-ion batteries analysed by operando solid-state NMR

“…Our group observed the overcharged states of graphite and hard carbon electrodes in LIB (full cells) using in situ 7 Li NMR in previous reports. 19,20,30 We observed a ''relaxation effect'' of the negative electrodes after overcharging, that is, a decrease of the signal of Li metal deposited on the negative electrode surface. The effect was observed intensely in the first few hours, and the electrodes reached an equilibrium state after 8-15 h. 19 Thus, it is also important to evaluate the behaviour of the Li in the negative electrodes after overlithiation in the overcharged state.…”

Review—Promises and Challenges of In Situ Transmission Electron Microscopy Electrochemical Techniques in the Studies of Lithium Ion Batteries

“…The morphologies can be imaged by SEM, 59,[65][66][67][68][69][70] TEM, [70][71][72][73][74][75][76] and atomic force microscopy (AFM). 77,78 The structures can be acquired by XRD, 76,78 extended X-ray absorption fine structure (EXAFS), 79,80 neutron powder diffraction (ND), 81 nuclear magnetic resonance (NMR), 59,82 and spherical aberration-corrected scanning transmission electron microscopy (STEM). 83,84 Elements contained in LIBs can be detected by inductively coupled plasma (ICP), 85 secondary ion mass spectroscopy (SIMS), 86 X-ray photoelectron spectroscopy (XPS), 87 electron energy loss spectroscopy (EELS), 83,84,88 scanning transmission X-ray microscopy (STXM), 89 X-ray absorption near-edge structure (XANES), 80 and X-ray fluorescence (XRF).…”

Long-run in operando NMR to investigate the evolution and degradation of battery cells

“…For the electrical connections, meshes attached to the electrodes have been generally used [11][12][13][14][15][16][17] . Conductive foils for the connections are possible as well, but sealing is known to be more challenging [18][19][20] . Bag cells are flexible and low-cost.…”