What is the interface reaction between non-carbon anode and electrolyte?

What is the interface reaction between non-carbon anode and electrolyte?

One disadvantage of commercial graphite is the limited capacity (theoretical capacity of 372 mAh/g) prior to LiC6 formation. A lithium battery with a metal oxide as the positive electrode will deposit lithium metal on the graphite surface when the battery is overcharged. At the same time, it will promote the decomposition of the electrolyte and the release of gas in the battery. These security issues are not conducive to its practical application. Therefore, it is necessary to find alternative anode materials to solve problems including irreversible capacity loss during the first charge-discharge process. Now, Si/Sn/Sb-based metals, alloys, and carbon-based compounds are all considered as anode materials to replace carbon materials. These materials exhibit high theoretical capacities, even as high as several thousand mAh/g. However, their first irreversible capacity loss is large and the capacity is difficult to maintain during long-term cycling. For more information on battery anode materials, please click here to open.

Different kinds of compounds and alloys containing Si, Sn, and Sb are being developed, but there are few studies on their electrolyte interfacial chemistry and SEI film analysis as anode material systems. The alloying and dealloying of lithium is an irreversible reaction accompanied by a large degree of lattice volume change, which is far more disordered than the intercalation and de-alloying reactions of lithium ions in carbon materials. Different from carbon materials, the reaction between alloy substances and organic solvents has a catalytic effect, and the catalytic effect varies with the alloy composition. The extreme change in particle size caused by the reaction with lithium within the alloy electrode will cause the alloy to crack. Therefore, ensuring the mechanical flexibility of the protective SEI film is important for battery performance.

The volume change and particle pulverization during the first cycle can cause the electrode-electrolyte interface reaction area to increase, so Si/Sb-based alloys are prone to self-discharge [31, 32]. Self-discharge involves the electrode-electrolyte electron transfer leading to the gradual decomposition of the electrolyte and the release of lithium ions from the negative electrode, which occurs even without the application of any current and voltage.

Analysis of the surface composition after cycling shows that the interfacial chemistry between metal and electrolyte is quite different from graphite (see Figure 1). The surface groups of LixCu2Sb are -CH2CH3, -COO-ester group, saturated ester-COOR group (with alkyl bond), -CO2-hydroxyester group in the presence of lithium oxalate salt Li2(O2C)2, lithium succinate salt Li2(O2CCH2)2, -C-O ester group in LiOCH3 [33]. Other components are inorganic materials such as Li2CO3 and -P-F- groups. While some of these surface functional groups are similar to those on graphite surfaces, there are additional decomposition products that are catalyzed by the alloy.

Figure 1-FTIR spectrum of SIE layer on the surface of Cu2Sb electrode
Figure 1-FTIR spectrum of SIE layer on the surface of Cu2Sb electrode

The XPS analysis results showed the existence of LiF, Li2CO3, Li2O and a small amount of polymer on the surface of the Sn-Sb-Cu-graphite alloy anode after cycling [34].
Li4Ti5O12 is a typical oxide-based anode material. It is an ideal zero-strain material with a stable crystal structure, which is not affected by the intercalation and deintercalation of lithium ions. The material is very different from metals and alloys that have large volume changes. However, to apply this material, it is necessary to replace the cathode material and electrolyte to adapt to the high working voltage of 1.5 V. Despite the above shortcomings, Li4Ti5O12 holds great promise for improving the stability and safety performance of Li-ion batteries. Due to the relatively high working voltage of Li4Ti5O12, which is 0.9 V higher than the reduction and decomposition potential of LiPF6/EC/PC electrolyte, SEI film will not be formed without interfacial reaction. When the LiBOB-containing electrolyte decomposes at about 1.75 V, an SEI film may be formed.

Figure 2 - Reductive decomposition of lithium salts in electrolyte a) Lipe6 and b) Libob
Figure 2 – Reductive decomposition of lithium salts in electrolyte a) Lipe6 and b) Libob

As shown in Fig. 2, the reduction potential peak of LiPF6 is at 1.5 V, but the reduction peak increases to 1.75 V when LiBOB is included in the electrolyte [35]. This suggests that SEI films may form on Li4Ti5O12 surfaces using LiBOB and other additives that decompose above 1.5 V.

Read more: What are the properties of polymer electrolytes?

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