Development of high-performance flexible solid-state micro-battery

A standalone all thin-film lithium-ion battery can be readily thinned down to achieve flexibility. The elimination of any dead volume from both the substrate and encapsulation contributions achieves the highest energy stored in a volume. The lithium-ion battery (Fig. 2a, left panel) consists of an active battery stack built on bulk monocrystalline (100) silicon (Si) substrate, which serves as a sacrificial host layer supporting the thin film stack during complementary metal oxide semiconductor (CMOS) process handling. Complete removal of Si substrate from the battery’s back (Fig. 2a, middle panel) results into a standalone robust but physically flexible active stack (30 µm total thickness), comprising of an insulation layer of silicon dioxide (SiO 2 ), a thick cathode current collector of aluminum (Al), a lithium cobalt oxide (LCO) cathode as the main source of lithium ions, a glass-like solid state lithium phosphorous oxynitride, titanium (Ti) as the anode current collector and lastly hermitically sealed top protective layers (Fig. 2a, right panel). Each individual battery with an active area of 2.25 × 1.7 mm micro-cell, deploys a “lithium-free” lithium-ion battery construction26 in which the anode plays the same role as the current collector, avoiding the dangers associated with reactive lithium metal. Such small size cells are attractive for miniaturized implantable flexible systems. The fast, transfer-less process significantly improves previous work27 for substrate removal, which entails the usage of an etching reagent in the final steps (see Methods and Supplementary Fig. S1). Xenon difluoride (XeF 2 ) dry etching is the ideal etchant for Si because it etches isotropically (i.e., independent of the crystal orientation) and resolves chemical, thermal, and mechanical compatibility considerations with battery materials. In order to effectively terminate the etching process, the surface roughness is monitored via atomic force microscopy (AFM), as it appears in Fig. 2b. We find at the end of the 50th cycle that the battery back surface morphology is flat with the absence of any surface roughness. This indicates the complete consumption of all the Si with only silicon oxide (SiO 2 ) and aluminum (Al) remaining as the bottom layers due to its very high selectivity (1000:1) to XeF 2 gas when compared to Si. The complete etching by the end of the 50th cycle is obtained for a total etching time of 36 min, at an etch rate of 67 nm/s. Thickness reduction along different cycles observed using Zygo profiler gives the value of the consumed etched volume plotted against the number of the etching cycles (Fig. 2c). This corresponded to an etching efficiency of 0.18 (see Methods for more details). Consequently, in order to verify the complete consumption of the Si substrate, the flexible battery’s back surface was analyzed using high-annular dark-field scanning tunneling transmission electron microscopy (HAADF-STEM) and a linear elemental analysis dispersive spectroscopy (EDS) scan. Figure 2d shows two contrasts for the last layers in the stack of a flexible battery belonging to Al and SiO 2 . This indeed confirms the complete consumption of the Si substrate with sufficient protection of the battery active materials during dry etching.

Fig. 2 Flexing process for a bio-compatible flexible lithium-ion battery. a SEM images of a bulk thin-film Si-supported battery (left panel; 130 μm total thickness) compared to thinned flexible battery with complete removal of Si support (middle panel; 30 μm total thickness). Cross section SEM image of the flexible thinned battery components made from all solid-state materials (right panel; lithium-free anode battery configuration). b Progressive Si back surface evolution of the battery undergoing selective dry etching as observed in a 15 × 10 μm2 AFM mapped region of the battery’s backside at zero etching cycles (left panel; roughness from wafer grinding), and with completed etching at 50 cycles (right side panel; flat surface and complete Si consumption). c Etching efficiency calculation through determination of the total etched volume using optical profiler measured across different etching cycles. d Confirmation of Si removal from the battery’s backside using elemental linear scan EDS analysis with the corresponding HAADF-STEM image (inset image) Full size image

Our flexing process for thin-film-based micro-batteries achieves two major objectives: (a) utilization of mature and reliable CMOS process28 with 90% yield and repeated electrochemical measurements on multiple devices (Fig. 3a–c and Supplementary Fig. S2), (b) ability to withstand high annealing temperatures of cathode material or soldering that are unachievable using direct film deposition on plastic substrates. Comparable flat voltage profile exhibited in both original bulk battery and the flexible version is observed during galvanostatic testing in the voltage plateau between 3 to 4.2 V, even after 120 charging/discharging cycles. For the 1st cycle as shown in Fig. 3a, the nominal voltage of the battery is 3.9 V with a discharge capacity of 146 μAh/cm2 at a current density of 130 μA/cm2 (discharge current of 5 μA). Different current ratings (2 C) 235 μA/cm2, (1 C) 130 μA/cm2 and (0.5 C) 68 μA/cm2, gives a variation in the discharging capacity of 148, 146, and 135 μAh/cm2, respectively. This is due to internal losses from polarization and mass diffusion limitations as observed for other current ratings (Supplementary Fig. S2). The resultant volumetric energy density (200 mWh/cm3) achieved in this work gives one of the highest values ever reported for a thin film all solid-state micro-battery as summarized in Supplementary Fig. S3.

Fig. 3 Mechanical and electrochemical properties of the battery. a Galvanostatic charging and discharging of bulk (blue) and flexible battery (red) in the 1st and 120th cycle at room temperature and 1 C current rating. b Discharge capacity and calculated coulombic efficiency over 120 cycles during charge and discharge cycles for bulk (blue) and flexible battery (red) with a limiting charge voltage at 4.2 V and a discharge cutoff voltage at 3 V. c Finite element analysis (FEA) of bulk and flexible battery under 10 mm bending showing simulated less strain simulated values for flexible battery without Si than the bulk battery with Si d battery under bending optical image and cross section SEM image of flexible battery under 1 mm bending radius. Zoomed-in photo of fracture-free active cathode battery material showing absence of micro-cracks. e Capacity retention under various current ratings 1 C, 2 C, and 0.5 C, respectively, at room temperature for a flexible battery under two conditions; (red) flat with no bending and (green) under 2.5 mm bending. f TEM diffraction pattern image of LCO cathode material exhibiting preferred orientation or texturing and assigned from EDS radial intensity profile (see Supplementary Fig. S6 for peak assignment). Crystal structure of LCO and correlating texturing with lithium-ion diffusion shows that (1 0 4) orientation exhibit higher diffusivity yet strain along the c-axis may increase diffusivity in (0 0 3) oriented crystals Full size image

The discharging capacities and calculated columbic efficiency of the battery during 120 cycles shows a capacity retention up to 70%. The main drop is observed (Fig. 3b) during the 2nd charging cycle compared to the 1st charging cycle, which is attributed to the intercalation mechanism of lithium-free lithium-ion battery described by the redox reaction LiCoO 2 ↔ Li 0.5 CoO 2 + 0.5 Li. During the first charge, lithium-ion leaves the LCO lattice and deposits on the Al cathode current collector. Yet, not all lithium ions will be available for subsequent intercalation reactions and a considerable amount will remain as a deposited lithium on the current collector. Whereas, the stable yet slight drop in the columbic efficiency is mainly due to the formation of solid-state interface during further cycling.

Thickness reduction impacts the overall mechanical stability of a flexible battery. Figure 3c shows comparative finite element analysis (FEA) of a bulk and flexible battery material stacks relates the thickness to the applied stress for a 10 mm bending. Since the thickness of a flexible battery without silicon is around 30 µm, then radius, we definitely get lower strain value (~0.19%, five times less) than with silicon. Estimated values of the strain for each material in the stack assess the material integrity within the elastic limit (Supplementary Fig. S4). This is followed by experimental verification of the strain effect on a flexible battery in both bent and unbent states (Fig. 3c, d). A radius of 1 mm is considered as the worst case scenario of operation. No observed effect on the microstructure of the stack or formation of micro-cracks, specifically to the active LCO cathode material and contact between active battery materials as seen in Fig. 3d and Supplementary Fig. S5. It is worth mentioning that all the characterization done is without any packaging consideration in order to demonstrate the highest volumetric energy at ambient conditions. A difference in capacity between flexible batteries under no bending and under bending in general favors the performance of the battery bent under 2 mm bending radius as seen in Fig. 3e even at different current ratings. To verify this, the value one must notice that the capacity in a thin-film battery generally depend on the diffusion barrier and morphology of LCO layer. LCO cathode material examined with selected area diffraction pattern transmission electron microscopy image shows the presence of polycrystalline rings, which is an expected microstructure in LCO films prepared using radio frequency sputtering. LCO is a rhombohedral (symmetry group, \({\rm{R}}\overline 3 {\rm{m}}\)) two-dimensional structure with a hexagonal lattice consisting of layers of closely packed oxygen atoms separated by alternating layers of lithium and cobalt (Fig. 3f). Rotational average and radial intensity profile, with dominant peaks identified dominant texturing is (0 0 3) planes approx. ± 70 degrees from growth direction and (1 0 4) planes approx. ± 90 degrees from growth direction are analyzed in detail in Supplementary Fig. S6. The two-dimensional structure in LCO thin films offers only two-diffusion paths for Li-ion intercalation. In the (1 0 4) orientation, lithium ions are nearly perpendicular to the substrate. Therefore, lattice planes are almost parallel to direction of travel of the ions. Leading to expected superior charging and discharging characteristics with flat profile. Examining the (0 0 3) orientation restricts lithium ions. In this orientation, lithium ions cannot intercalate easily and lithium-ion diffusion across such grains is only restricted along grain boundary because Li ions are trapped within the stack. Diffusion barrier considerations is another factor to facilitate diffusion for the (0 0 3) oriented grains. Fanghua et al.29 density functional calculations (DFT) have concluded the effect of LCO strain in the reduction of the lithium diffusion barrier.29 Since tensile strain reduces the barrier due to large interlayer and in-plane spacing, facilitating the movement of lithium ions along the c-axis. Strain along c direction is achievable since the majority of the LCO grains in our film preferably orient themselves along the (0 0 3). Therefore, for both crystal orientation and diffusion barrier effect, our flexible battery under bent state shows a larger capacity than its flat condition.

Any implantable device must be tested for toxicity before usage. In that regard, cell culturing is one of the main methods to test bio-compatibility.30 HEK cell culture grown on batteries over days showed healthy proliferation behavior. Based on light and fluorescence cell culture images, low number of cells was attached to the battery surface on 1st day of incubation (Fig. 4a); however, after 3 (Fig. 4b) and 5 days (Fig. 4c) of incubation, more healthy colonies were strongly adhere with high coverage (stained in green). On the other hand, dead cells (stained in red) are present in a low coverage on the surface. The dramatic increase in cell number over days indicates the bio-compatibility of the battery surface. Furthermore, the cell viability test (CCK8) in Fig. 4d, showed a high percentage of HEK viable cells reaching to approximately 90% when incubated with the battery. Clearly, the two different cell viability assays depict the safety profile of the test battery, which confirms its bio-compatibility property.

Fig. 4 Biocompatibility and thermal environmental testing for flexible battery. Progressive HEK 293 cellular growth on the top surface of the non-cytotoxic battery identified by conventional optical and fluorescent microscope images recorded at different times: a day 1, b day 3, and c day 5. (Left panels shows optical microscope images; right panels shows fluorescence microscope images). Large green stains corresponds to the growing number of living cells compared to the trivial red stains corresponding to dead cells. d CCK8 cell viability assay compares cell culture density on the flexible battery with a control group after 5 days of testing, which confirms the biocompatibility of the flexible lithium-ion battery. e Temperature effect on the operation of the flexible biocompatible battery compares the normalized discharge capacity at the first discharge cycle recorded for multiple flexible batteries heated at a designated temperature. The battery capacity increases with higher temperature as the battery’s internal resistance decreases. Thermal images of the battery are obtained during different temperatures Full size image

Fig. 5 Smart dental brace system. a Average minimalist radius of curvature can be obtained in dental arch is 10 mm. b Flexible battery module bonded to aluminum interconnects on PET substrate and its integration with chip-scale LEDs. The device is embedded in a 3D printed brace with battery and LED module repeated per each teeth. c Smart dental brace components: near-infrared LEDs integrated with flexible batteries and interconnected on a soft PET substrate. The whole device is embedded in semi-transparent 3D printed brace, d Flexible battery powering near-infrared two LEDs connected in series. e Image of the smart dental brace device on artificial teeth. f Top view of smart dental brace from the outside (left) and inside (right) with packaged red light therapy module Full size image

Since the normal body temperature is around 37 °C, the battery previously tested at room temperature must be retested at elevated temperatures to check its stability inside the human body environment. Electrochemical performance of the battery at elevated temperatures was done (Fig. 4e) to identify its safe usage. Temperature behavior of multiple batteries was examined on the first charge to isolate the effect coming from degradation of the batteries during cycling. The capacity observed to increase almost linearly as the temperature increases. This is normally attributed to the increased diffusion of lithium ions as the temperature increases.