Lithium Transport of Fast Battery Cycles in a LiMn2O4 Cathode Imaged By Operando Eels
Authors: Lee Soyeon , Oshima Yoshifumi , Suzuki Kota , Kanno Ryoji , Hosono Eiji , Zhou Haoshen , Takayanagi Kunio 
Published: February 2020

Lithium ion batteries are required to be fast-chargeable. However, it has been reported that fast charge causes low reversible capacity and fast degradation of electrode materials [1]. Recently, in-situ observation of lithium ion batteries has been reported that phase transformation behaviour of cathode materials changed depending on charge rate. In-situ XRD studies showed that meta-stable phase appeared during fast charge, but not during slow charge in LiFePO4 cathode [2]. Our previous in-situ TEM studies on LiMn2O4 cathode showed that during fast charge-discharge two-phase reaction continued to the voltage range of solid-solution reaction [3]. Also, it was imaged that Li-excess tetragonal phase appeared at the interface between cathode and electrolyte during early stage of 4 V range [4]. These results suggest the different lithium transport mechanism of fast charge from that of slow charge. However, XRD and TEM methods monitor the structural change, not lithium ion itself. Direct Li imaging method would be very helpful to reveal the lithium transport behaviour during fast charge.

Electron energy loss spectroscopy (EELS) is one of most suitable methods to detect lithium ion itself because Li-K edge peak due to the existence of lithium ions can be detected. Furthermore, recent quantitative analysis of EELS can visualize the lithium concentration change simply [5]. In this study, we performed operando EELS mapping for understanding the lithium transport by using our developed nano battery, which consists of LiMn2O4 nanowire cathode, ionic liquid electrolyte and Li4Ti5O12 anode.

 The nano battery was charged or discharged in TEM during operando EELS observation as loaded in our homemade in-situ TEM sample holder. Charge-discharge was performed by cyclic voltammetry of which scan speed was 0.55mV/s. EELS spectrum was recorded by Gatan Tridiem ER spectrometer (Gatan, Inc.) in aberration corrected TEM, R005 [6] which is equipped with cold field emission gun. The acceleration voltage was 200kV and the convergent semi-angle of electron probe was 23 mrad. The entrance aperture size was 60 mrad and the energy dispersion of 0.2eV/ch was used. The full width half maximum of zero loss peak was 1 eV. The pixel size of EELS images was around 9 x 9 nm2. The electron probe current was less than 5 pA in order to suppress electron beam damage. The dwell time per pixel was 0.2 s and the probe size was adjusted to pixel size.

During fast battery cycles, we found that lithium concentration changed with time delay with the cell current change in both of charge and discharge. Furthermore, the lithium concentration was imaged to change from the area far from the cathode/electrolyte interface. The observed lithium concentration change cannot be explained by conventional diffusion model, but the lithium drift model that lithium moves due to electric potential gradient inside cathode caused by high charge rate. In addition, after the first battery cycle lithium concentration changed only slightly around the range of x~1 in LixMn2O4 near the interface. It is attributed that solid electrolyte interface formation at high voltage after charge makes the chemical potential changed locally as an interface effect. We believe that these findings open the new way to understand the phase transformation under non-equilibrium state such fast charge as well as the new strategy to design fast chargeable batteries.

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