Most Li-ion batteries currently are several sub-types currently manufactured Li-ion batteries in which different cathode materials, anode materials, or electrolyte type are used. Depending of what the battery is going to be used for, the users must take into account several important parameters, or “figures of merit” – namely:
- Gravimetric energy density – i.e., how much energy can be stored in a battery of unit mass, usually expressed in the units of Wh/kg);
- Volumetric energy density – energy per unit volume, usually given in Wh/liter);
- Power density (maximum power that can be drawn from a battery of unit mass or unit volume – respectively, in W/kg or W/liter);
- The “lifetime” – i.e., the guaranteed number of charge/discharge cycles at which the energy density will stay above a certain threshold usually 80% of the original density;
- Charging time – the shortest recharging time the battery can tolerate (too fast a charging leads to overheating and damage to the battery);
- The charge/discharge rate4 in the so-called “C units”;
An even more comprehensive list of important parameters can be found in this MIT Web page.
For EV makers, for instance, the gravimetric energy density and the charging time are highly important. For aircraft makers who want to install Li-ion batteries in a new model, the safety is of utmost importance. For a not-too-wealthy retired hobbyist working on a small project the cost may be a meaningful factor.
Below the main existing types of Li-ion batteries are discussed.
Graphite anodes + 3d transition metal oxide cathodes. The “3d transition metals” are those with numbers from 21 to 30 in the Periodic Table (the “3d” in the name does not mean “three dimensions”, but refers to the 3d electronic orbitals in their atoms). The first Li-ion cells created by John Goodenough and coworkers in the 1980-s had intercalation cathodes made of LixCoO2 (commonly referred to as “Lithium Cobalt Oxide”, or LCO) – as described in the text above. Cobalt, however, is expensive and raises some moral objections – namely, most of it comes from the Democratic Republic of Congo, in which “democracy” can be found primarily in its name, but not much in the functioning of the state. In particular, human rights are not respected in the cobalt mines, where the use of child labor is commonplace. In search for less controversial substitutes of cobalt, researchers started experimenting with some other transition metal oxides. One intercalation host that was found to be a good cathode material was Lithium Manganese Oxide (LMO). Many other efforts were focused on mixtures, or alloys of several 3d metal oxides which did not eliminate cobalt completely, but needed much less of it. A number of such cathodes from oxides have been developed, such as, e.g., Lithium Nickel Manganese Cobalt Oxide (NMCO), or a composition with the addition of aluminum (Al), Lithium Nickel Cobalt Aluminum Oxide (or NCAO). Aluminum does not belong to the family od 3d transition metals, but a certain fraction of Al in the oxide mixture has been found to result of a record-high performance of such cathode. The batteries used in the Tesla cars have NCAO cathodes and a record-high energy density of 265 Wh/kg. A comprehensive review of Li-ion battery sub-types, with the discussion of the “figures of merit” listed above for each sub-type, is presented in this Web article.
Graphite anode + Lithium Iron Phosphate(LiFePO4) cathode. In 1996 scientists from the University of Texas discovered a viable non-oxide based cathode material, LiFePO4 (LFP). The energy density of batteries with such cathodes is only about one-half of that of the best batteries with oxide-based ones, but the offer several meaningful advantages – e.g., higher tolerance to heat and therefore they are much less likely to catch fire than the other types. Also, they experience much slower degradation, their “lifetime” can reach thousands of charge/recharge cycles. Yet another advantage are their high charge/discharge rates, higher than those typically seen in cells with oxide cathodes.
The nominal voltage of a LFP cell is 3.2 V (slightly lower than the 3.7-3.8 V of Li-ion cells with oxide cathodes). Four of them make a 12.8 V battery that – due to a high discharge C rate – may offer a “lightweight replacement” for a 12 V lead-acid one typically used in automobiles. With one caveat, though – lead batteries in cars are constantly recharged with 14.4 V, which they tolerate very well while in phosphate iron, such a regime leads to excessive charging which quickly damages them. Therefore, such batteries should be installed in cars only with additional electronics controlling the charging process.
Also, due to their reliability and tolerance for “harsh environment”, Iron phosphate batteries are especially recommended as replacement of lead-acid batteries in marine applications, in yachts and small vessels.
Batteries with Lithium Titanate anodes (Li4Ti5O12). They are the first commercially available Li-ion batteries with a non-graphite anode. Even though their capacity is only about the same as that of nickel metal hydride batteries – in other words, not impressively high – but they are well suited for certain special applications due to their exceptional tolerance to low temperatures, and their very high charge/discharge rates.
Lithium polymer (LiPo) batteries. The name is a bit misleading because the LiPo-s are essentially Li-ion batteries using the same anode and cathode materials as the “regular” Li-ion ones. The main difference is in the type of electrolyte used. The “regular” cells use a porous separator material soaked with liquid electrolyte. In the Lithium-polymer cells the electrolyte has the form of a thin film made of a gelled ion-conducting substance. Such an electrolyte effectively protects against “short-circuiting”, i.e., a direct contact between the cathode and anode. Thus, no separator is needed as an additional element. Some commercially available LiPo batteries are packed not in metal capsules, but in flexible plastic envelopes that one can bend within certain limits Without fear that the internal structure will be damaged.
Trends in research on new types of L-ion batteries.
Lithium metal anodes. In the early efforts of creating lithium-ion batteries anodes made of bulk lithium metal, not of intercalated lithium atoms were used. Those attempts were unsuccessful, mainly because during the recharging process lithium ions were not deposited uniformly on the flat anode surface, but instead formed “three-like structures” called dendrites. After several charge/discharge cycles the dendrites created shortcuts between the anode and the cathode, which not only rendered the battery useless, but might even set in on fire. The dendrite-growing process is well illustrated in this short video made in the Oak Ridge National Laboratory.
Recently, attempts of using pure lithium metal anodes have been renewed. The stake is high! Why? Let’s keep in mind that in the currently used graphite anodes in a fully charged state there is one Li atom per each six carbon atoms. The atomic weight of Lithium is 7, and of carbon it is 12. Hence, for a fully charged anode, LiC6, the total molecular weight is 6 x 12 + 7 = 79. The fraction of lithium in it is thus 7/79 = 8.9% of the total weight. Accordingly, replacing the intercalated material by pure lithium would reduce the weight of the anode more than tenfold! Even if all other component of the battery are not changed, the anode of pure lithium may nearly double the gravimetric energy density of the battery. The total mass of the battery pack in the famous Tesla car is 540 kg and if fully charged, it can propel the car over a distance of 400 miles. A 540 kg package of batteries with lithium-metal anodes would nearly double that range! So, as the old proverb says, the game is worth the candle! Scientist, in this recent article, are investigating several possible ways of suppressing the dendrite growth – one of them may be, for instance, using not a liquid, but a “solid state electrolyte” sturdy enough that the dendrites would not be able to penetrate it. John Goodenough, the inventor of the lithium-ion battery, recently announced that a team led by him had developed such an electrolyte.
Silicon anodes. Silicon can become “lithiated” – not through intercalation, but through another physicochemical mechanism it can bind as many as 3.75 lithium atoms per one silicon atom, while graphite can host one Li atom per six carbon atoms. The atomic weight of silicon is 28. Hence, if a silicon crystal is in a “fully lithiated” state the, the fraction of lithium in it is (3.75 7)/(3.75 7+ 28) = 48.4% by weight – more than five times a better figure than the 8.9% for graphite. So, it raises hopes that silicon may become the anode material in next generations of “lithium silicon batteries”. However, Mother Nature has a perverse sense of humor – the volume of a fully lithiated silicon is 300% of that of the pure element (for comparison, the volume of graphite increases by only 10% when it is saturated with lithium). Yet, the lithium-silicon battery concept is so attractive that researchers are trying to design it in such a way that the increase in anode volume would not interfere with its operation – see, e.g., this report.
The Lithium-Sulfur battery. In 2008, the British BBC agency reported a new sensational world record: namely, a pilotless solar powered aircraft Zephyr-6 stayed aloft for three days at an altitude of 60 000 feet. What? An overnight flight of a solar-powered aircraft? Yes, it was possible, thanks to a new ultra-light rechargeable battery pack. During daylight hours the solar panels on wings yielded enough power to operate the engines and to recharge the battery – with enough energy to keep the plane aloft overnight.
The new sensational battery had the energy density twice as high as the best li-ion ones available in 2008. A metallic lithium anode and a sulfur cathode were used. In later years the record was significantly improved with the plane remaining in the air for 11 days. The energy density of the battery pack used was 500 Wh/kg, almost two times better than the 265 Wh/kg of the best Li-ion cells used in the Tesla cars.
So why aren’t these lithium-sulfur batteries widely used in electric cars?
They could increase the range of the Tesla auto to almost 800 miles which would probably satisfy even the most demanding customers. Unfortunately, they are still haunted by problems. The most serious is that they offer too few charge/discharge cycles. For current electric vehicles the required battery lifespan is at least 500 charge/discharge cycles. The Li-S cells made in laboratories sometimes attain that many, but a technology that would make it possible to manufacture them on industrial scale with required parameters has still not been mastered. Some optimistic news have been recen.tly heard from Australia, and a British company Oxis Energy promises to manufacture Li-S rechargeable batteries with 600 Wh/kg within five years. Let’s keep our fingers crossed and watch the news!
Further reading: for a deeper understanding of the physicochemical processes occurring in lithium-ion cells, it is worth recommending the papers linked below. They present, in advanced manner, both an approach based on macroscopic thermodynamic theory in terms of Gibbs free energy, as well as an approach based on a detailed analysis of microscopic electron states in electrode materials in the light of quantum theory of conducting solids.
• ChaofengLiu et al. (2016).
4The battery capacity can be expressed in terms of energy units – but another, perhaps even more common measure are ampere-hours (A h). A capacity of N A h means that a fully charged battery will be completely discharged by delivering a current of N Amps for one hour. The charging/discharging rate unit “C is simply related to the A h capacity measure. For instance, if a battery of, say, 10 A h capacity has a charging rate of 0.5C, it means that the maximum charging current for it should be 0.5*10 A = 5A. A discharge rate of 5C would mean that the maximum current safely drawn from this battery cannot exceed 5*10A = 50 A.