Improved Electrochemical Performance of an All Solid-State Microbattery By Electrodeposition of Polymer Electrolyte into the Nanostructured Electrodes
Authors: Salian Girish , Lebouin Chrystelle , Djenizian Thierry 
Published: February 2020

The miniaturization of Lithium ion batteries (LIBs) as a power source to drive small devices such as smartcards, medical implants, sensors, radio-frequency identification (RFID) tags etc. has been continuously developed to meet the market requirements of portable applications1. In this direction 3D microbatteries have been considered to satisfy the requirements of these portable devices. All solid-state microbatteries involving self-supported titania nanotubes (TiO2 nts) have been considered, due to their high surface area and short diffusion lengths for Li+ ion transport2. It has been reported that the electropolymerization of the polymer electrolyte through cyclic voltammetry (CV) leads to the conformal deposition of the electrolyte into the 3D TiO2 nts 3,4. So far, the electropolymerization and its influence on the capacity was reported with regard to TiO2 nts only5. This work, along with electropolymerized TiO2 nts, includes the electropolymerization of the polymer electrolyte onto the Lithium nickel manganese oxide (LNMO) composite cathode and its influence on the capacity of the whole microbattery. A drop cast layer of the monomer methyl methacrylate-polyethylene glycol (MA-PEG) serves as the separator and ion conducting medium. Fig.1 a) and b) shows the CV of the electropolymerization of the polymer electrolyte into the TiO2 nts and LNMO respectively. The cathodic current density decreases with cycle number which indicates that the thin polymer layers is deposited onto the TiO2 nts wall which helps in using the effective surface area of the nanotubes. For the LNMO composite electrode, the higher current density observed is due to the high rugosity of the cathode layer. This suggests that the polymer layer formed at the initial cycles allows to follow the rugosity of the LNMO electrode, which is important to establish a good electrode-electrolyte interface. A comparison of the capacities by electropolymerized electrodes (TiO2nts(EP)/Polymer/LNMO(EP)) and non-electropolymerized electrodes (TiO2nts/Polymer/LNMO) shows the positive influence of the electrodeposition of the polymer electrolyte on the capacity of the microbattery which is shown in Fig. 1 c) and d). TiO2nts(EP)/Polymer/LNMO(EP) microbattery delivers a capacity of 169 mAh.g-1 (82 μAh cm−2 μm−1) at the first cycle and 150 mAh.g-1 (70 μAh cm−2 μm−1 ) for the tenth cycle at C/10 rate. Comparison of the capacities at C/10 and C/2 with TiO2nts/Polymer/LNMO showed an increment of around 100% even after 100 cycles.6

Fig. 1 Cyclic voltammetry of the electropolymerization on the (a) TiO2nts and (b) LNMO in aqueous solution of 0.5 M MMA-PEG + 0.5 M LiTFSI vs. Ag/AgCl (saturated) in the potential window -0.35 V to -1 V at a scan rate of 10 mV.s-1. Galvanostatic charge/discharge profiles of different microbatteries at C/10 for (c) TiO2nts(EP)/Polymer/LNMO(EP); (d) TiO2nts/Polymer/LNMO.
References
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