Stabilizing Crystal Framework of an Overlithiated Li1+xMn2O4 Cathode by Heterointerfacial Epitaxial Strain for High-Performance Microbatteries

To meet the increasing demands of high-energy and high-power-density lithium-ion microbatteries, overlithiated Li1+xMn2O4 (0 ≤ x ≤ 1) is an attractive cathode candidate due to the high theoretical capacity of 296 mAh g–1 and the interconnected lithium-ion diffusion pathways. However, overlithiation triggers the irreversible cubic-tetragonal phase transition due to Jahn–Teller distortion, causing rapid capacity degradation. In contrast to conventional lithium-ion batteries, microbatteries offer the opportunity to develop specific thin-film-based modification strategies. Here, heterointerfacial lattice strain is proposed to stabilize the spinel crystal framework of an overlithiated Li1+xMn2O4 (LMO) cathode by epitaxial thin film growth on an underlying SrRuO3 (SRO) electronic conductor layer. It is demonstrated that the lattice misfit at the LMO/SRO heterointerface results in an in-plane epitaxial constraint in the full LMO film. This suppresses the lattice expansion during overlithiation that typically occurs in the in-plane direction. It is proposed by density functional theory modeling that the epitaxial constraint can accommodate the internal lattice stress originating from the cubic-tetragonal transition during overlithiation. As a result, a doubling of the capacity is achieved by reversibly intercalating a second lithium ion in a LiMn2O4 epitaxial cathode with a complete reversible phase transition. An impressive cycling stability can be obtained with reversible capacity retentions of above 90.3 and 77.4% for the 4 and 3 V range, respectively. This provides an effective strategy toward a stable overlithiated Li1+xMn2O4 epitaxial cathode for high-performance microbatteries.

The geometry relationship between the d-spacing of ( 400) and ( 404) planes of the LMO structure (d400 and d404) is illustrated for a bulk LMO unit cell in Figure S8a.The measured d400 and d404 values, can be related to the in-plane axis by the following equations: where  represents the angle between ( 004) and ( 404

Figure S1 .
Figure S1.(a) XRD patterns of pristine 100-LMO and activated 100-LMO films (discharged 3.6 V); (b) Mn 2p XPS spectra of pristine 100-LMO film; (c) Initial three CV curves of activation process for 100-LMO film.Peaks from Nb-SrTiO3 and SrRuO3 are marked with ♥ and ♣, respectively.♦ identifies the minor contribution of Mn2O3 impurity phase.The y-axis in (a) is log-scale.

Figure S3 .
Figure S3.(a) XRD patterns of pristine 110-LMO and activated 110-LMO films (discharged 3.6 V); (b) Initial three CV curves of activation process for 110-LMO film.Peaks from Nb-SrTiO3 and SrRuO3 are marked with ♥ and ♣, respectively.The y-axis in (a) is log-scale.

Figure S4 .
Figure S4.The raw FFT images of (a) 100-LMO film and (b) 110-LMO film that correspond to the yellow box areas in Figure 2a and 2c.

Figure S5 .
Figure S5.HAADF-STEM images of the (a) 100-LMO/SRO and (d) 110-LMO/SRO heterointerfaces; Line profiles of the (b) Mn1 column and the (c) Sr or Ru column along [011] direction in (a); Line profiles of the (e) Mn1 column and (f) Sr and Ru column along [11 ̅ 0] direction in (d).Length of the scale bar: 1 nm.

Figure S6 .
Figure S6.The schematic crystal structures of (a) LMO and (b) SRO along [100] zone axis; crystal structure of (c) LMO and (d) SRO along [110] zone axis.Blue and yellow boxes indicate the Mn columns and Sr columns, respectively.
) planes of LMO unit cells.Given the fact that the in-plane crystal planes of the bulk spinel structure are symmetric, the distorted in-plane axes should be undergoing the same compressed strain.Therefore, the b and c-axes of 100-LMO films are symmetric along the [011] zone axis (FigureS8b) while the a and b-axes of 110-LMO films are symmetric along the [110] zone axis (FigureS8c).

Figure S9 .
Figure S9.The oxidation peaks of a 100-LMO film at (a) 3 V range and (b) 4 V range; The reduction peaks of a 100-LMO film at (c) 3 V range and (d) 4 V range.

Figure S10 .
Figure S10.The oxidation peaks of a 110-LMO film at (a) 3 V range and (b) 4 V range; The reduction peaks of a 110-LMO film at (c) 3 V range and (d) 4 V range.

Figure S12 .
Figure S12.The charge/discharge profiles of (a-d) 100-LMO films and (e-f) 110-LMO films with different thicknesses.The applied current density for all films is 80 µA cm -2 .7200 and 14400 indicate the applied pulses during the PLD deposition process.

Figure S14 :
Figure S14: Reproducibility studies of 100-LMO films (S1, S2 and S3 indicated three individual 100-LMO film).(a) The CV curves at 1.0 mV s -1 ; (b) The rate and cycling performance.Although variations in CV curves and reversible capacities are observed and they are attributed

Figure S16 .
Figure S16.The average bond lengths of MnO6 octahedra and dO-O values along b and c direction for bulk overlithiated Li2Mn2O4 calculated by DFT modelling.The DFT calculated lattice parameters for the bulk overlithiated Li2Mn2O4 are b=c= 5.68 Å and

Table S1 .
Summary of quantified capacity (presented by absolute in Coulombs) of the films in the 4V region, 3V region and middle region which are determined by first-order derivative curves shown in FigureS12.The capacities of additional 2 nd and 3 rd 100/110-LMO-14400 films are also shown to confirm that the measurements about capacity are statistically sound.