Pyrite In Batteries: An Introduction
With the global push away from fossil fuels to renewable sources of electricity well underway, the question of energy storage comes to the fore. Recent research shows the potential of pyrite as a material in batteries, allowing potential production of high performing battery cells without the need for excess expensive or hard-to-use materials such as cobalt and cadmium.
Battery Cells And Pyrite
A traditional battery cell is a simple electrochemical cell comprising a cathode, an anode and some kind of electrolyte between them. When a load is applied to the opposing electrode ends (forming a circuit), a current begins to pass, using the stored energy. Common battery electrodes include lithium and sodium. Batteries are measured in terms of their discharge capacity and cycle capacity, that is, how effective the cell is at charging or discharging over a number of cycles. The ability to charge, discharge and recharge is critical in modern battery design for both environmental and efficiency related concerns.
Pyrite has enjoyed lots of attention owing to the fact that it is an n-type semiconductor. It has a band gap of 0.9 eV, yet has never been hugely successful in the area of solar cells in its own right(1). This could be partly explained by the fact that n-type semiconductors have electrons as their charge carriers, whereas in p-type, the charge carriers are holes - and p-type semiconductors are the most common in solar cells. This may, however, be advantageous in battery scenarios. Not least because pyrite is plentiful.
One concern of the use of pyrite as an electrode in a battery cell is the potential to form metal sulfides in a conversion reaction that may take place under certain circumstances in situ in the cell, particularly in rechargeable cells with carbonates present:
Fe2S + 4M → 2M2S + Fe
Where M is one of the metals lithium (Li) or sodium (Na). The conversion from metal to metal sulfide will inevitably result in a lower battery efficiency owing to reduced conductivity. Various strategies may be employed to prevent this, described throughout, with specific examples in the section ‘Enhancing pyrite’s performance’. This transformation is referred to throughout the literature as the ‘conversion reaction’.
Lithium Batteries And Pyrite
Lithium containing batteries are perhaps the most popular and most promising battery-type electrical storage medium at the current time. Lithium is relatively common and cheap. Using pyrite is attractive because it too is relatively inexpensive. As with all technology, removing excess cost enables effective scale-up of production and therefore lower costs for the end consumer.
Solid state lithium batteries based on the gravimetric energy conversion associated with sulfur represent a promising new battery chemistry, allowing for supreme charge densities, resilience and a suitability for where high positive electrode loadings are needed. In these cells, carbon - iron disulfide - sulfur composites are employed as positive electrode materials, with a conventional lithium anode. Pyrite is used as a source of iron, iron disulfide and sulfur, reached via ball milling and heat treatment. Research has shown such solid state cells can bear charge densities of around 1,200 mAh g-1 at low loadings, reaching 3,500 mAh g-1 at high loadings(2).
One concern with solid state lithium cells bearing pyrite is that of output voltage, with numbers for pyrite cells often existing in the region of 1 to 3 V, further reduced by the potential for the sulfur and lithium sulfide reaction to be sluggish. Improvements to this have been suggested via the inclusion of catalytic amounts of cobalt, improving the electrochemical performance of pyrite significantly(3). Additionally, the doping with catalytic cobalt can improve capacitance overall by up to 90%(4).
Other battery types, such as gel, have been shown to be compatible with pyrite as a key component. With hybrid electrolytes and a pyrite electrode, researchers have found that longevity is afforded to the system via the inclusion of pyrite, with a Li/FeS2 cell degrading (in terms of cycling performance) by only 0.1% per cycle(5). It has been shown that across certain cell types, the contamination of the pyrite surface by iron hydroxides, oxides and sulfides can impact performance of the pyrite electrode, albeit these effects are reversible and/or short lives(6).
Sodium Batteries And Pyrite
Compared to lithium batteries, there is much less of a developed body of research into sodium batteries with pyrite. It should be noted that sodium is as economically attractive as lithium. It has been reported that iron pyrite has the highest charge carrying capacity amongst iron-based cathodes for sodium ion cells(7). Discharge levels in the region of 630 to 758 mAh g-1 are common for the simple Na/FeS2 system, which is theoretically 4 Na ions per pyrite, representing an efficiency of up to some 85% based on this theoretical capacity. Like lithium cells containing pyrite, sodium cells benefit from good cycling capability with only modest decreases in efficiency after repeat uses(8).
Looking towards more complex systems employing pyrite alongside sodium, researchers have shown that rechargeable batteries are well within the preserve of pyrite. Using pyrite microspheres in a cell reduces the effects of the ‘conversion reaction’ and ensures that the cell retains circa. 90% efficiency even after 20,000 cycles - suggesting long term use opportunities. Additionally, the researchers showed that the technology was easily scalable to almost 19,000 units - meaning easy commercialisation(9). In an even more complex setup, pyrite composited with carbon were produced as a fibre using the electrospinning method. Combining this with sodium deposited on a polyimide film, a cell was created that in addition to possessing one of the best reported charge/discharge performances (ca. 850 mAh g-1), has the added benefit of being physically flexible(10).
Enhancing Pyrite’s Performance
Whilst using pyrite in an electrochemical cell such as a battery often increases performance in and of itself, there are other methods that can be used to further increase efficiency. One such method is the pre-treatment of a pyrite electrode with dimethyl sulfoxide solution and/or a mixture of dimethyl sulfoxide and gelatin - both in a non-aqueous way. When used with a typical LiPF6 electrode, the reversible specific charge capacity of the overall cell was increased over tenfold compared to a pristine, untreated, pyrite electrode(11).
Other modification methods include a combined solvo-chemical synthesis and annealing process to afford nanospherical pyrite, which can be deployed as an electrode(12). Researchers have shown that electron transfer (i.e. conductivity) is increased by the material possessing a defined nanostructure - displaying capacities exceeding 650 mAh g-1 with capacity retention of 97% after 100 cycles. This particular treatment method for high surface area electrode production may be especially well suited to rapid charge and discharge application lithium ion batteries. Other work in the nanochemistry area has looked into “decorating” a framework of pyrite with carbon, providing a high efficient cathode for lithium cells(13). In developing this type of structure, the authors claim that it helps deal with the large variation in volume relative to conductivity experienced across many types of lithium ion cells.
There is some concern, however, with possible problems when using pyrite as an electrode in lithium batteries. These derive from potential agglomeration of Fe0 and low conductivity of resultant lithium sulfide. Researchers have shown that modification of pyrite such as via calcination or forming into a nanochemical structure alleviates these issues(14).
One final concern specific to using nanoscale pyrite in the production of lithium and sodium ion batteries is the effect of solid-electrolyte interface layer effects. In broad terms, such effects increase the likelihood of irreversible conversion reactions (for example deposition of sulfides, reducing conductivity). Researchers have shown, however, that ultrafine nanoscale pyrite (around 4.5 nm) can be used with no limitation(15).
- Pyrite has a long history in batteries and electronics, owing to its nature as an n-type semiconductor, with a small band gap of just 0.9 eV
- Lithium batteries containing pyrite is a promising example of new battery chemistry. Pyrite can be included in a lithium cell of several types, often affording the system excellent charge capacity and recharhability
- Sodium cells with pyrite seem to be a less studied area, but pyrite can enhance the ability of a simple sodium or sodium ion cell by similar means as with lithium cells. One of the added benefits of the sodium pyrite system is its exceptionally low cost
- Because of pyrite’s unique semiconducting ability (in that sometimes it isn’t a great conductor at all), many have taken to using non-standard treatments of pyrite components or looking towards the material’s battery enhancing properties on the nanoscale
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