LIB design and preparation

Cathode electrodes were prepared from a mixture of a Teflon binder, (e.g. –(CF 2 ) n –), carbon particles as conductive diluent (e.g., acetylene carbon black), and active LiMn 2 O 4 (NEI, Grade BE-30) powder in a weight ratio of 20:5:75 %, respectively and the assemblage was optimized. The binder keeps particles in laminate form, the carbon provides a conductive network amongst the particles, and active material oxide acts to support the electrochemical reaction and act as lithium cation host. After replenishing with isopropanol, the mixture was well-mixed and ground with a mortar and pestle until it formed a self-standing laminate. A thin sheet of laminate was achieved by manual rolling, after which it was dried at 75 °C in an oven and stored under vacuum. The free standing electrode was obtained by placing the pre-cut laminate (14 mm in diameter) onto an Al mesh (14 mm in diameter), then the laminate and the mesh were pressed together under hydraulic pressure (Carver Laboratory Equipment). The top of a 2032 coin cell was modified by punching a hole (8 mm in diameter) and then sealing the hole with Kapton tape prior to coin cell assembly. The modified coin cell was assembled with the cathode laminate, a glass fiber separator (Whatman® Glass microfiber filters, Grade GF/F), GenII electrolyte (1.2 M LiPF 6 in 3:7 EC-EMC solution, Tomiyama Pure Chemical Industries), and a lithium metal anode (FMC Lithium) in an Argon-filled glove box. To probe an ‘open’ light-accepting coin cell battery, the tape was removed from the hole and a transparent quartz window (Technical glass products Inc.) was glued into place over the hole.

Light and electrochemical experiments set up

For the ‘light’ experiments, a 300 W Xenon lamp (Atlas Specialty Lighting with a Perkin Elmer power supply) that spans from 300 nm to 1100 nm was used as a white light source and an IR filter (Newport) was used to avoid undesired heating of the cell. The spectral output of the Xe lamp used in the work has <5% overall energy emitted in the UV region. The incident light was normal to the sample and the temperature of the cell was measured by an IR thermometer focused on the transparent window. There was a temperature increase of ~7 °C when electrochemical measurements were carried out under ~1 SUN condition (100 mW cm−2). The heat effect in control experiments are presented in Supplementary Fig. 10. These results separate the heat contribution to the potentiostatic current due to thermal activation compared to photo-activation. Heating the ‘open’ cell at 35 °C (10 °C higher), increases the charge current by a factor 1.16 (not doubled), and at 45 °C, a factor of 1.26 was realized over that of 25 °C in “light-off” condition.

Cycle performance of the modified cells and of conventional (un-modified) cells were compared by measurements involving galvanostatic charge and discharge of the cells between 3.2 and 4.4 V at various C rates ranging from 2 C to C/10 (calculated based on a theoretical capacity of 148 mAh g−1 for LiMn 2 O 4 ). The active LiMn 2 O 4 material in an electrode typically weighed between 20 and 25 mg. Chronoamperometry was carried out by using a Gamry and Solartron SI1260 Frequency Response Analyzer. ‘Light-on’ and ‘light-off’ state chronoamperometry measurements were performed by holding a constant voltage of 4.07 V (vs. a Li metal counter electrode). All relevant data processing/analysis was done in Matlab (Mathworks Inc., Natick).

Raman spectroscopy

Raman measurements were performed using a Renishaw inVia Raman microscope equipped with a 785 nm excitation laser. A 1200 l/mm grating was used in conjunction with a Ren 578 CCD detector. The laser power delivered to the sample surface was < 2 mW; the laser spot diameter was approximately 10 µm. Spectra were recorded through the window of the cell (BaF 2 or quartz) using a 50-X objective with a numerical aperture of 0.5. A total of ten co-added 20 second exposures were made at each of the probed spots in the rectangular map. Other details of the Raman measurements and the spectra processing/analysis are the same as presented in refs. 25,30,31,32.

Absorption and transient absorption measurements

Absorption measurements were carried out using a Cary 5000 UV-Vis-NIR spectrophotometer. The UV/Vis absorption spectrum of the electrolyte was measured and was found to be transparent over the visible region of the spectrum, with minimal absorption in the UV portion. With little light absorption across this spectral region, it is accurate to predict no overt degradation of the electrolyte under long term light exposure.

Spectrally-resolved transient absorption (TA) measurements were performed using a Helios EOS spectrometer and an amplified Ti:sapphire laser operating at 1 kHz that was tuned to produce 390 nm pump pulses using an optical parametric amplifier. White light probe pulses were derived from a picosecond Nd:YAG laser and photonic crystal fiber. A portion of the probe pulse was beamsplit to produce a reference laser shot for noise reduction. Pump-probe delay time was controlled and evaluated electronically. For TA measurements, two samples were used (a LMO only film as a control and LMO composite electrodes used in fully operational ‘open’ cells). For the LMO film preparation, a ~ 200 nm film of LMO was deposited on a quartz substrate (Technical Glass Products, USA) with a commercial sputtering system (AJA International, USA). Deposition was conducted by RF magnetron sputtering of a stoichiometric LMO target (>90% density) in argon at 3.1 mTorr and room temperature. The net RF power was 75 W and the deposition rate was ~6 Å min−1. A lithium overpressure was provided by DC sputtering from a Li-metal target at 15 W set off axis by 20°. The deposition times were 9 hours for ~300–350 nm films. The as-deposited LMO films were found to be amorphous; however a subsequent anneal air at 800 °C for 16 h resulted in a crystalline/polycrystalline films.

Electron paramagnetic resonance (EPR) experiments

Continuous wave (CW) X-band (9–10 GHz) EPR experiments were carried out with a Bruker ELEXSYS II E500 EPR spectrometer (Bruker Biospin, Rheinstetten, Germany), equipped with a TE 102 rectangular EPR resonator (Bruker ER 4102st). A helium gas-flow cryostat (ICE Oxford, UK) and an ITC503 from Oxford Instruments, UK, were used for measurements at cryogenic temperatures (T = 10 K). Light excitation was done directly in the resonator with a 300 W Xenon lamp (LX 300 F from Atlas Specialty Lighting with PS300–13 300 W power supply from Perkin Elmer). A water filter was used in combination with a KG2 short pass filter (Schott) to avoid unwanted heating of the sample. A GG400 long pass filter (Schott) was used to remove UV light. Data processing was done using Xepr (Bruker BioSpin, Rheinstetten) and Matlab 7.11.1 (The MathWorks, Inc., Natick) software. Simulations were performed using the EasySpin software package (version 5.0.20).

Density functional theory (DFT) calculations

We carried out our atomistic calculations using the plane-wave density functional theory (DFT) code Quantum-Espresso33. Electron-nuclei interaction was taken into account by using recently developed ONCV norm-conserving pseudopotentials34,35,36. The wave function energy cutoff was 80 Ry. We used a unit cell with formula Li 8 Mn 16 O 32 and 4 × 4 × 4 k-point sampling. The cell was optimized at the PBE + U level of theory with a U Mn = 3.5. Previous studies showed that taking into account Jahn-Teller distortion and using hybrid or DFT + U levels of theory are essential for describing the charge localization that accompanies the two different oxidation states of Mn15,37. On average, the oxidation state of Mn is 3.5, which turns to formally 3 and 4, when the cubic unit cell is Jahn–Teller distorted into a tetragonal structure. It was also demonstrated that ferromagnetic (FM) and antiferromagnetic (AFM) arrangements of Mn ions are very close in energy, although the AFM ordering was shown to be slightly more stable15. More importantly, the optical properties were similar with AFM and FM ordering, we thus restricted our studies to FM ordering to avoid the complications arising from the multiple possible magnetic solutions when searching for the disproportionated state. To ensure the robustness of the results, we also performed PBE0 hybrid functional calculations. It was shown for many semiconductors that the mixing fraction entering the PBE0 hybrid functional should be chosen as the inverse of the high frequency dielectric constant. For LMO, the dielectric constant is 4.78 at the HSE06 level of theory15, suggesting that the original PBE0 mixing fraction of 0.25 might give rise to reliable quasiparticle band gaps. PBE0 calculations were carried out at the DFT + U geometry with a Monkhorst-Pack (MP) grid of 2 × 2 × 2, a momentum transfer grid of 1 × 1 × 1 and a reduced 80 Ry energy cutoff for the Fock operator. For the PBE0 calculations, we used a locally modified version of Quantum-Espresso to start PBE0 calculations with PBE + U wave functions. This not only allowed us to reach convergence much faster than from a PBE starting point, but it also made sure that the right magnetic state is reached. We found that the energy difference between the ground state and the disproportionated state was 0.72 eV at the PBE0 level of theory, which is in good qualitative agreement with that obtained with DFT + U (0.45 eV). See Supplementary Fig. 9 for a comparison of the DOS obtained at the DFT + U and PBE0 levels of theory.

We monitored oxidation states and local magnetizations using atomic projected charges and spin densities in QE and then verified the results by using a Maximally Localized Wannier Function (MLWF) approach. In particular, we computed MLWFs in QBOX using the same pseudopotentials that we used in QE38. We used the PBE0 hybrid functional, a reduced MP grid of the Gamma-point only and a wave function energy cutoff of 60 Ry. Since LMO is a strongly ionic system, the Wannier centers are located in close proximity to atoms. We thus counted the number of Wannier centers close to each atom and defined the oxidation state as the number of valence electrons minus the number of Wannier centers associated with that atom. The magnetic moment was computed by subtracting the number of up spin Wannier centers from the number of down spin Wannier centers. This approach of assigning oxidation states to atoms in solids is similar to what has been proposed in the literature by Jang et al.39, where the oxidation state was defined using the number of Wannier centers that move with an atom if the atom is displaced by a lattice vector39,40.

As described in the main text, the ground state structure of bulk LMO contains alternating and ordered Mn3+ and Mn4+ sites. We first constrained an excess electron on several different Mn3+ sites (making the chosen site Mn2+) with the excess electron being spontaneously withdrawn from another Mn3+ site (to become Mn4+). Having optimized the structure under this charge constraint, we further re-optimized the structure with the constraint released. We obtained the disproportionated structure by constraining the charge at a Mn site. We tried several different methods to constrain an excess charge: (i) increasing/decreasing the U of the selected Mn atom, (ii) charging the supercell, (iii) constraining the Mn-O bond lengths to ~2.2 Å (see Supplementary Fig. 10) for the Mn–O bond lengths in different Mn oxidation states). At the end, we found the disproportionated structure by a combination of (i) and (ii).