Optimization of Anode Slurry Preparation and Its Performance Evolution in Lithium-Ion Batteries

Abstract

The electrochemical performance of lithium ion battery (LIB) mainly depends on structural integrity of active materials and stability of electrode/electrolyte interface. The theoretical values such as cell potential, specific capacity and the energy density of LIB depends on characteristics of the active material. The practical values of specific capacity, energy density, cycle life and rate capability depends on the extrinsic properties such as electrode microstructure i.e., adhesion and cohesion of the electrode, coating thickness and its porosity and the interfaces between various components of the cell. Aim of this work is to improve the electrochemical performance of the cell by controlling the properties of electrode by changing the sequence of addition of the constituents during slurry preparation. Further, effect of composition and thickness of electrode on electrochemical properties were investigated.In the present work, five different sequence of addition (Fig.1) of slurry constituents were studied. All slurries in this study were prepared using high speed vacuum shear mixer. Method-I: graphite and carbon black were added in sequence to the PVDF-NMP binder solution. Method-II: Hand mixed composite of carbon black and graphite powder was added together to the PVDF-NMP binder solution. Method-III: pre-mixed dry composite of graphite and carbon black was added to the PVDF-NMP binder solution. Method-IV: graphite and PVDF was added in sequence to the mixture of carbon black-NMP suspension and in Method-V: carbon black followed by graphite was added to the PVDF-NMP binder solution. This comprehensive study was carried out to understand the effects of sequential addition on the rheological properties of the slurry such as viscosity from flow curve and viscoelastic modulus from the amplitude sweep test. Viscoelasticity is the measure of degree of flocculation and the type of network structure in the slurries.All slurries best fitted for Herschel-bulkley model of shear-thinning behaviour. Storage modulus of all the slurries was in the range of 100-500 Pa. The electrode was fabricated by coating the slurry (prepared by five methods) on copper current collector by doctor blade technique. SEM (Scanning electron microscope) and elemental analysis have been carried out to understand the homogeneity in the distribution of active/in-active materials.Coin-cells (Graphite vs Lithium metal) were fabricated with electrode prepared by five methods and galvanostatic charge/discharge cycles has been carried out at 1C rate. The specific capacity of the anodes at 1C-rate were in the order of 340(II)> 314(IV) > 300(I)> 233(III)> 210 (V) mAh/g. Specific capacity of the electrode prepared by method-II exhibited superior cyclic stability. The capacity retention was around 94% even after 200 charge/discharge cycles. Impedance analysis of electrodes were carried out after every five charge/discharge cycles at 0.1C rate with frequency range of 0.01Hz-1MHz and observed that charge transfer resistance were in the order of Method-II> Method-IV> Method-I> Method-III> Method-V. Electrode prepared by Method-II has lower charge transfer resistance of 20Ω and less polarization compared to the electrode prepared by other methods.Further, the effect of electrode composition (active material and inactive material) and electrode thickness on electrochemical characteristics were studied by using method-II. The variation in electrochemical performance with respect to electrode composition and thickness will be discussed in detail during the presentation. Figure 1