Influence Of Conductive Networks On Electrochemical Characteristics Of Lithium Iron Phosphate Cathode

Carbon materials,due to their high conductivity and stability,are used as conductive additives in lithium-ion batteries,making them an important component that significantly influences battery performance.In this study,lithium iron phosphate（Li Fe PO₄,LFP）was used as the active material to construct the electrode,and the influence of different-sized carbon black（SP）conductive additives on electrode conductivity and electrochemical performance was investigated.Furthermore,the conductivity of carbon black was compared with that of new conductive additives,namely vapor-grown carbon fibers（VGCF）and graphene（GN）,to observe the influence of different types of conductive additives on the electrochemical performance of LFP electrodes.Finally,a binary composite of SP,VGCF,and GN carbon materials was used as the conductive additive in LFP electrodes,and the influence of the composite conductive additive on electrode performance was explored.This research provides valuable insights for the selection of conductive additives in the practical production process for companies.First,two types of carbon black particles with similar degrees of graphitization and defect levels,having average particle sizes of 68 nm and 50 nm（referred to as JSP and XSP,respectively）,were compared in terms of their effects on the performance of LFP electrodes.It was found that the LFP electrode with JSP as the conductive additive（LFP-JSP）exhibited a specific capacity of 136.1 m Ah·g^-1 with a capacity retention rate of 99.5%.Whereas,the LFP electrode with XSP as the conductive additive（LFP-XSP）showed higher discharge specific capacity and better cycling stability(after 100 charge-discharge cycles,the discharge specific capacity of LFP-XSP was 138.8 m Ah·g^-1 with a capacity retention rate of 99.8%).This improvement was primarily attributed to the larger surface area of the smaller XSP particles,which increased the contact points between the conductive additive and the active material,thereby reducing electrode resistance.However,the particle size of the conductive carbon black had an impact on the electrode compaction density.The increased number of contact points between the smaller XSP particles and the electrode material hindered the achievement of high electrode compaction density,consequently affecting the utilization of the battery’s volumetric energy density.Secondly,the influence of conductive additives with different structures on the performance of lithium iron phosphate（LFP）electrodes was compared.The study found that the particulate conductive additive SP had uneven distribution within the electrode material,and the point contact between particles resulted in a large contact resistance.Therefore,the increasing effect of SP on electrode conductivity is not significant.By contrast,the one-dimensional linear VGCF and two-dimensional sheet-like GN formed a continuous conductive network by connecting the LFP particles,which facilitated electron transport and significantly improved the conductivity of the electrode（the electrodes with added SP,VGCF,and GN conductive agents were respectively named LFP-SP,LFP-VGCF,and LFP-GN）.After 100 cycles at a 1 C rate,the oxidation-reduction peak voltage differences for the LFP-VGCF and LFP-GN electrode systems were 0.5 V and 0.273 V,respectively,with polarization voltages lower than the LFP-SP electrode system（0.686 V）.This demonstrates that VGCF and GN conductive additives effectively enhance the electrochemical reaction activity of the LFP electrode.Finally,SP-VGCF,SP-GN,and VGCF-G composite conductive additives were individually added to LFP to prepare LFP electrodes（named LFP-SV,LFP-SG,and LFP-VG,respectively）,and the influences of the composite conductive additives on the electrochemical performance of LFP electrodes were studied.The results showed that the resistivity of the three composite conductive additives electrode systems was superior to that of the electrode systems with a single conductive additive added（resistivity of LFP-SP,LFP-VGCF,and LFP-GN electrode systems were 275Ωcm,263Ωcm,and 188Ωcm,respectively）,resulting in a significant increase in specific discharge capacity at low rates but no significant difference in discharge performance at high-rates.Among them,the LFP electrode prepared with VGCF conductive agent and GN conductive agent had the lowest resistivity,only 44.7Ωcm at a compacted density of 1.8 g·cm^-3.The excellent composite conductive network resulted in a significantly lower resistivity of LFP-VG electrode compared to LFP-SV electrode and LFP-SG electrode（resistivity of 230.3Ωcm and 184.4Ωcm,respectively）.At a 1 C rate,the discharge specific capacities of LFP-SG,LFP-SV,and LFP-VG after 100cycles were 139.9 m Ah·g^-1,140.8 m Ah·g^-1,and m Ah·g^-1,respectively,all higher than the discharge specific capacities of LFP-SP,LFP-VGCF,and LFP-GN single conductive agent electrode systems at 1 C rate(135.2 m Ah·g^-1,137 m Ah·g^-1,and 135.8m Ah·g^-1,respectively).This demonstrates that the synergistic effect between the composite conductive agents is beneficial for improving the electrode conductivity and obtaining better electrochemical performance.