Reversible Li Insertion in Guest Free Type II Si Clathrates for Li-Ion Batteries
Authors: Dopilka Andrew , Weller Jon Mark , Bobev Svilen , Chan Candace K. 
Published: February 2020

Stable intercalation anodes for Li-ion batteries are comprised of two groups: graphite and Ti/Nb oxides. Graphite electrodes are reversible but can’t be operated at high current rates because of particle fracture and dendrite formation as a result of volume expansion and a low reaction potential (0.1 V). Li4Ti5O12 is a popular insertion anode that demonstrates good stability (zero strain) and fast charging but its high reaction voltage (1.55 V) lowers the energy density of the full cell. A crystal framework that allows low strain Li-insertion at a lower potential (i.e. 0.2 -1.0 V) could allow for fast charging with less propensity for dendrite formation while also increasing the energy density of the cell. Intermetallic compounds have reaction voltages with Li between 0.2 V – 1.0 V, which makes them interesting candidates for insertion anodes. However, Li alloying accompanied by a large volume expansion is common in these materials (e.g. Si, Ge, Sn) so host frameworks that allow bulk Li insertion into the structure before a Li alloying reaction are needed.
Intermetallic clathrates are crystal structures that are comprised of a group IV framework of cages which host alkali guest atoms in the center of the cages (e.g. Ba8Si46, K8Ge44). This structure type has led to many interesting materials properties such as thermoelectricity, superconductivity, hydrogen storage, and tunable optical properties. Recently, our group has been studying the electrochemical reactions of clathrates with Li to understand how the defects and characteristics of clathrates affect the electrochemical properties and if reversible Li insertion into the crystal structure is possible. Our previous work on Type I clathrates with filled cages indicate that guest atoms are frustrating Li mobility and that bulk Li diffusion between cages requires guest atom vacancies1,2.
Type II Na-Si clathrates (NaxSi136 (0< x < 24)), composed of Si20 and Si28 cages (Figure 1a), are well known for having tunable guest atom occupancy based on the thermal treatment. Previous work has proven with nuclear magnetic resonance spectroscopy that Li can be inserted into the empty clathrate cages of Si136 when the guest atom occupancy is low (x = 0-1)3. This insertion process manifests as a potential plateau at 300 mV prior to the amorphization of the Si lattice. Lithiation of the clathrate cages has been demonstrated but delithiation and cycling of Si136 has not been investigated. In addition, the Li pathways and positions in the lithiated Si136 structure have not been concluded.
In this work, the synthesis and electrochemical and structural characterization of guest free type II Si clathrate is reported. The type II Si clathrates (Si136) were synthesized via thermal decomposition of Na4Si4 under vacuum and then characterized with synchrotron powder XRD and transmission electron microscopy to confirm the low Na content. By applying a voltage cutoff (~260 mV) prior to the onset of the amorphization reaction, reversible Li insertion is demonstrated for the first time (Figure 1b). The delithiation shows uniquely asymmetric behavior indicating a different delithiation mechanism than from the lithiation. The Si136 structure can insert ~32 Li before beginning the Li alloying reaction which corresponds to a capacity of around 220 mAh/g (470 Ah/L). When cycled with a voltage cutoff prior to the Li alloying reaction, the electrode demonstrates a repeatable electrochemical profile demonstrating the reversible Li chemistry. In addition, we find that the capacity depends sensitively on the voltage cutoff used (Figure 1c). After 50 cycles, the electrode maintains the clathrate crystal structure, evidence of a reversible insertion reaction. Ex-situ XRD of the fully lithiated clathrate (Figure 1d), shows that the clathrate structure has a very low volume expansion of around 0.1%, indicating that the lithiation process could be considered zero-strain. Synchrotron powder XRD and X-ray pair distribution function analysis is used to confirm the positions of Li during insertion and elucidate the lithiation mechanism. Since the Li insertion voltage is 300 mV for Si136, the risk of dendrite formation at higher current rates could be reduced while maintaining higher energy density cell due to a low reaction voltage. These results are interesting for the future design of insertion anodes that maintain a higher full cell energy density similar to that of graphite while still providing rapid and reversible Li insertion.

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Figure 1