Personal computers and mobile phones are perhaps the most obvious examples of portability, a key attribute of some of the most common devices in use today. Yet such devices still depend on a not-particularly-modern type of technology – batteries.

By far the most common type is the lithium-ion unit. These can be made at small sizes to enable their use in handheld applications, but also be scaled up for more intensive applications such as powering electric vehicles.

Rechargeable lithium-ion batteries work by maintaining a flow of ions from the negative electrode to the positive electrode during use. The ion flow is reversed during charging. But these batteries suffer from a consistent inhibiting factor, namely, their need for a liquid electrolyte material as the medium through which the ion flow takes place. Although liquid electrolyte materials are well-established, they suffer from a number of drawbacks: low charge retention and poor performance at extremes of both high and low temperatures among the leading faults.

(Click to enlarge.) Timeline of Li-on battery development. Image source: Green Car Congress.As such the prospects for completely solid alternatives might be considerable if appropriately reliable high-performance electrolyte materials could be formulated. But so far that has not been the case. Instead the solid-state batteries that have been developed have also generally proven unsatisfactory not least because the solid electrolyte materials they use have had a higher resistivity than liquid counterparts. What’s more, some of the formulations have proven chemically unstable. As a consequence they have only been found in small-scale, highly specialized applications such as heart pacemakers.

(Learn more about lithium batteries, including links to products and suppliers, at Engineering360.)

Now, however, that situation could be set to change. A team of researchers in Japan has announced that they have created two lithium-based crystalline “superionic” materials that can act as solid electrolytes without suffering either from the inefficiencies or cost penalties associated with earlier attempts to achieve the same objective. The development team included members from three Japanese organizations, the Tokyo Institute of Technology, the High Energy Accelerator Research Organization and Toyota Motor Co.

Prof. Ryoji Kanno says that the new cells are relatively easy to produce. (Image source: Tokyo Tech)According to Prof. Ryoji Kanno of the Tokyo Institute, the new materials are derived from previous work that was first reported in 2011. That research resulted in a solid electrolyte formulation that made use of the relatively expensive constituent germanium, which meant that even if practicable units could be developed they would have been prohibitively costly. The earlier research did, however, prove a landmark because that initial material was, he says, “the first which showed higher lithium ionic conductivity than organic liquid electrolyte.” Its precise chemical formula (Li₁₀GeP₂S₁₂) also provided a generic name “LGPS” for what he says is a new family of materials with a distinctive internal structure that allows for the easy flow of ions.

The new materials promise to break through barriers that have inhibited the performance both of previous experimental solid materials and existing commercial liquid electrolytes. The materials are called, respectively because of their chemical formulations, LSPSC (Li₉.₅₄Si1.₇₄P₁.₄₄S₁₁.₇Cl₀.₃) and LPS (Li₉.₆P₃S₁₂). As part of the research, the Japanese team subjected their crystal structures to scrutiny using both a synchrotron X-ray diffractometer and a neutron diffractometer.

Kanno says that compared with previous solid materials the first exhibits superior ionic conductivity, and the second has a more enhanced chemical stability. These characteristics enabled the researchers to construct two different cells. In the first case, they constructed one with high current drain capability. In the second case, the cell had a high capacity. Both cells helped to demonstrate the different properties involved.

Comparing the cells’ performance with existing liquid electrolyte materials may be particularly exciting. For instance, the cells were found to operate consistently well over a temperature range of -30C to +100C. They also exhibited high energy and high power densities with small internal resistance levels. Importantly, the team also reported that the cells’ properties would allow them to be stacked close together without interference, enabling construction of multi-cell battery packs.

Furthermore the cells also exhibited fast charging, excellent charge retention and a long lifespan. After more than 500 cycles, they still retained around 75% of their initial discharge capacity, the researchers say.

Meanwhile, another potentially attractive attribute of the materials is that their actual manufacture seems to be relatively straightforward. Kanno says that the experimental quantities produced so far were synthesized by a “conventional sintering technique.” Although he concede that mass production may pose issues, he says these should not be particularly problematical. “Basically they are easy to produce,” he says.

Early indications about ease-of-disposal after use also appear promising. Kanno says that the materials are soluble in water and that, as such, their disposal should be “no problem”. However, he also says that humidity can cause decomposition of the materials and generate hydrogen sulfide (H₂S) gas. But again a solution is likely to be fairly straightforward. The materials should be capable of being stabilized, he states, “by a small amount of additives.”

A possible timescale for full commercial development remains lengthy. “We hope all solid-state batteries may become available for commercial use in 10 years,” Kanno says. “However, we do not yet know whether it is possible because for the practical applications of large size batteries, processing technology such as sheeting and stacking is important.” Nevertheless the team is hopeful that their new materials will enable all-solid-state batteries for multiple applications, including electric vehicles.

More immediately, Kanno says that further work to develop a small-sized battery with even higher performance characteristics is envisaged and that “some companies have shown interest in our research.”